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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2019	Feb. 20, 2020	Jun. 28, 2019
Document And Entity Information
Entity Registrant Name	RIBBON COMMUNICATIONS INC.
Entity Central Index Key	0001708055
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Document Type	10-K
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		111,306,494
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 286,412,000

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 44,643	$ 43,694
Marketable securities	0	7,284
Accounts receivable, net	192,706	187,853
Inventory	14,800	22,602
Other current assets	27,146	17,002
Total current assets	279,295	278,435
Property and equipment, net	28,976	27,042
Intangible assets, net	213,366	251,391
Goodwill	224,896	383,655
Deferred income taxes	4,959	9,152
Operating lease right-of-use assets	36,654	0
Other assets	26,762	7,484
Total assets	814,908	957,159
Current liabilities:
Current portion of long-term debt	2,500	0
Revolving credit facility	8,000	55,000
Accounts payable	31,412	45,304
Accrued expenses and other	56,700	84,263
Operating Lease, Liability, Current	7,719	0
Deferred revenue	100,406	105,087
Total current liabilities	206,737	289,654
Long-term debt, net of current	45,995	0
Long-term debt, related party	0	24,100
Operating Lease, Liability, Noncurrent	37,202	0
Deferred revenue, net of current	20,482	17,572
Deferred income taxes	4,648	4,738
Other long-term liabilities	16,589	30,797
Total liabilities	331,653	366,861
Commitments and contingencies (Note 23)
Stockholders' equity:
Preferred stock, $0.01 par value; 10,000,000 shares authorized; none issued and outstanding	0	0
Common stock, 240,000,000 shares authorized, $0.0001 par value, 110,471,995 shares issued and outstanding at December 31, 2019; 106,815,636 shares issued and outstanding at December 31, 2018	11	11
Additional paid-in capital	1,747,784	1,723,576
Accumulated deficit	(1,267,067)	(1,136,992)
Accumulated other comprehensive income	2,527	3,703
Total stockholders' equity	483,255	590,298
Total liabilities and stockholders' equity	$ 814,908	$ 957,159

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	240,000,000	240,000,000
Common stock, shares issued (in shares)	110,471,995	106,815,636
Common stock, shares outstanding (in shares)	110,471,995	106,815,636

Consolidated Statements of Operations - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Total revenue	$ 563,111,000	$ 577,905,000	$ 329,942,000
Cost of revenue:
Total cost of revenue	246,027,000	269,573,000	128,446,000
Gross profit	317,084,000	308,332,000	201,496,000
Operating expenses:
Research and development	141,060,000	145,462,000	101,481,000
Sales and marketing	117,962,000	128,276,000	83,403,000
General and administrative	53,870,000	66,036,000	47,642,000
Impairment of goodwill	164,300,000	0	0
Acquisition- and integration-related	12,953,000	16,951,000	14,763,000
Restructuring and related	16,399,000	17,015,000	9,436,000
Total operating expenses	506,544,000	373,740,000	256,725,000
Loss from operations	(189,460,000)	(65,408,000)	(55,229,000)
Interest (expense) income, net	(3,877,000)	(4,230,000)	263,000
Other income (expense), net	70,444,000	(3,772,000)	1,274,000
Loss before income taxes	(122,893,000)	(73,410,000)	(53,692,000)
Income tax (provision) benefit	(7,182,000)	(3,400,000)	18,440,000
Net loss	$ (130,075,000)	$ (76,810,000)	$ (35,252,000)
Loss per share:
Basic (in dollars per share)	$ (1.19)	$ (0.74)	$ (0.60)
Diluted (in dollars per share)	$ (1.19)	$ (0.74)	$ (0.60)
Shares used to compute loss per share:
Basic (in shares)	109,734	103,916	58,822
Diluted (in shares)	109,734	103,916	58,822
Product
Revenue:
Total revenue	$ 262,030,000	$ 279,014,000	$ 181,119,000
Cost of revenue:
Total cost of revenue	133,347,000	142,185,000	70,250,000
Service
Revenue:
Total revenue	301,081,000	298,891,000	148,823,000
Cost of revenue:
Total cost of revenue	$ 112,680,000	$ 127,388,000	$ 58,196,000

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (130,075)	$ (76,810)	$ (35,252)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	194	220	(1,940)
Unrealized gain on available-for-sale marketable securities, net of reclassification adjustments for realized amounts	590	45	146
Employee retirement benefits	(1,960)	369	(578)
Other comprehensive (loss) income, net of tax	(1,176)	634	(2,372)
Comprehensive loss, net of tax	$ (131,251)	$ (76,176)	$ (37,624)

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive income (loss)	GENBAND	GENBAND Common stock	GENBAND Additional paid-in capital	Edgewater Networks, Inc.	Edgewater Networks, Inc. Common stock	Edgewater Networks, Inc. Additional paid-in capital	Anova Data, Inc.	Anova Data, Inc. Additional paid-in capital
Balance (in shares) at Dec. 31, 2016		49,041,881
Balance at Dec. 31, 2016	$ 219,122	$ 49	$ 1,250,744	$ (1,037,174)	$ 5,503
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan (in shares)		249,621
Issuance of common stock in connection with employee stock purchase plan	1,252		1,252
Exercise of stock options (in shares)		105,688
Exercise of stock options	617		617
Vesting of restricted stock awards and units (in shares)		2,160,553
Vesting of restricted stock awards and units	0
Vesting of performance-based stock awards and units (in shares)		145,357
Vesting of performance-based stock awards and units	0
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(807,952)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(7,523)		(7,523)
Shares issued as consideration in connection with acquisition (in shares)						50,857,708
Shares issued as consideration in connection with acquisition						$ 413,982	$ 5	$ 413,977
Reclassification between Common stock and Additional paid-in capital to record change in par value of common stock	0	$ (44)	44
Stock-based compensation expense	25,657		25,657
Other comprehensive income (loss)	(2,434)				(2,434)
Net loss	(35,252)			(35,252)
Balance (in shares) at Dec. 31, 2017		101,752,856
Balance at Dec. 31, 2017	615,421	$ 10	1,684,768	(1,072,426)	3,069
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		15,935
Exercise of stock options	73		73
Vesting of restricted stock awards and units (in shares)		1,278,062
Vesting of restricted stock awards and units	0
Vesting of performance-based stock awards and units (in shares)		57,768
Vesting of performance-based stock awards and units	0
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(524,516)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(2,024)		(2,024)
Shares issued as consideration in connection with acquisition (in shares)									4,235,531
Shares issued as consideration in connection with acquisition									$ 30,000	$ 1	$ 29,999
Assumption of equity awards in connection with acquisition of Edgewater Networks, Inc.									$ 747		$ 747
Stock-based compensation expense	10,013		10,013
Other comprehensive income (loss)	634				634
Net loss	$ (76,810)			(76,810)
Balance (in shares) at Dec. 31, 2018	106,815,636	106,815,636
Balance at Dec. 31, 2018	$ 590,298	$ 11	1,723,576	(1,136,992)	3,703
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan (in shares)		282,646
Issuance of common stock in connection with employee stock purchase plan	863		863
Exercise of stock options (in shares)		127,334
Exercise of stock options	235		235
Vesting of restricted stock awards and units (in shares)		1,504,707
Vesting of restricted stock awards and units	0
Vesting of performance-based stock awards and units (in shares)		9,466
Vesting of performance-based stock awards and units	0
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(240,673)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(1,193)		(1,193)
Shares issued as consideration in connection with acquisition (in shares)												2,948,793
Shares issued as consideration in connection with acquisition												$ 15,186	$ 15,186
Repurchase and retirement of common stock (in shares)		(975,914)
Repurchase and retirement of common stock	(4,536)		(4,536)
Reclassification of liability to equity for bonuses converted to stock awards	1,052		1,052
Stock-based compensation expense	12,601		12,601
Other comprehensive income (loss)	(1,176)				(1,176)
Net loss	$ (130,075)			(130,075)
Balance (in shares) at Dec. 31, 2019	110,471,995	110,471,995
Balance at Dec. 31, 2019	$ 483,255	$ 11	$ 1,747,784	$ (1,267,067)	$ 2,527

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (130,075)	$ (76,810)	$ (35,252)
Adjustments to reconcile net loss to cash flows provided by (used in) operating activities:
Depreciation and amortization of property and equipment	11,949	11,200	8,486
Amortization of intangible assets	49,225	49,723	17,112
Stock-based compensation	12,601	11,072	25,657
Impairment of intangible assets and goodwill	164,300	0	5,471
Deferred income taxes	5,299	513	(20,361)
Reduction in deferred purchase consideration	(8,124)	0	0
Foreign currency exchange losses (gains)	1,090	4,611	(783)
Other	0	0	(557)
Changes in operating assets and liabilities:
Accounts receivable	(3,936)	(13,017)	(30,759)
Inventory	7,776	993	5,786
Other operating assets	(17,489)	5,036	269
Accounts payable	(16,282)	(6,057)	13,415
Accrued expenses and other long-term liabilities	(18,538)	(13,422)	(4,263)
Deferred revenue	(2,111)	16,563	23,859
Net cash provided by (used in) operating activities	55,685	(9,595)	8,080
Cash flows from investing activities:
Purchases of property and equipment	(10,824)	(7,907)	(3,999)
Business acquisitions, net of cash acquired	0	(46,389)	(42,951)
Purchases of marketable securities	0	0	(28,731)
Sales/maturities of marketable securities	7,295	18,919	96,112
Proceeds from the sale of intangible assets	0	0	576
Net cash (used in) provided by investing activities	(3,529)	(35,377)	21,007
Cash flows from financing activities:
Borrowings under revolving line of credit	117,000	197,500	15,500
Principal payments on revolving line of credit	(164,000)	(162,500)	(13,500)
Proceeds from issuance of long-term debt	50,000	0	0
Principal payment of debt, related party	(24,716)	0	0
Principal payments of long-term debt	(1,250)	0	0
Payment of deferred purchase consideration	(21,876)	0	0
Principal payments of finance leases	913
Principal payments of finance leases		(652)	(99)
Payment of debt issuance costs	(891)	(624)	(731)
Proceeds from the sale of common stock in connection with employee stock purchase plan	863	0	1,252
Proceeds from the exercise of stock options	235	73	617
Payment of tax withholding obligations related to net share settlements of restricted stock awards	(1,193)	(2,024)	(7,523)
Repurchase of common stock	(4,536)	0	0
Net cash (used in) provided by financing activities	(51,277)	31,773	(4,484)
Effect of exchange rate changes on cash and cash equivalents	70	(180)	547
Net increase (decrease) in cash and cash equivalents	949	(13,379)	25,150
Cash and cash equivalents, beginning of year	43,694	57,073	31,923
Cash and cash equivalents, end of year	44,643	43,694	57,073
Supplemental disclosure of cash flow information:
Interest paid	4,072	2,367	317
Income taxes paid	4,665	5,505	2,290
Income tax refunds received	1,757	537	274
Supplemental disclosure of non-cash investing activities:
Capital expenditures incurred, but not yet paid	2,566	1,127	1,043
Property and equipment acquired under finance leases	1,442	2,178	0
Business acquisition purchase consideration - common stock issued	15,186	30,000	413,982
Business acquisition purchase consideration - deferred payments	1,700	30,000	0
Business acquisition purchase consideration - assumed equity awards	0	747	0
Business acquisition purchase consideration - note issued to selling equityholders	0	0	22,500
Supplemental disclosure of non-cash financing activities:
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested	$ 7,422	$ 8,312	$ 20,515

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The consolidated financial statements have been prepared in United States dollars, in accordance with accounting principles generally accepted in the United States ("GAAP"). On February 28, 2019 (the "Anova Acquisition Date"), the Company acquired the business and technology assets of Anova Data, Inc. ("Anova"). The financial results of Anova are included in the Company's consolidated financial statements for the period subsequent to the Anova Acquisition Date. On August 3, 2018 (the "Edgewater Acquisition Date"), the Company completed the acquisition of Edgewater Networks, Inc. ("Edgewater" and such acquisition, the "Edgewater Acquisition"). The financial results of Edgewater are included in the Company's consolidated financial statements for the period subsequent to the Edgewater Acquisition Date. On October 27, 2017 (the "Merger Date"), Sonus Networks, Inc. ("Sonus") consummated an acquisition as specified in an Agreement and Plan of Merger (the “Merger Agreement”) with Solstice Sapphire Investments, Inc. ("NewCo") and certain of its wholly-owned subsidiaries, GENBAND Holdings Company, GENBAND Inc. and GENBAND II, Inc. (collectively, "GENBAND") pursuant to which, following a series of merger transactions (collectively, the "Merger"), Sonus and GENBAND each became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. Subsequently, on November 28, 2017, the Company changed its name from "Sonus Networks, Inc." to "Ribbon Communications Inc." The consolidated financial statements of the Company represent the consolidated financial statements of Sonus, prior to the Merger Date, and the consolidated financial statements of Ribbon, on and after the Merger Date. The financial results of GENBAND are included in Ribbon's consolidated financial statements beginning on the Merger Date. Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements include the accounts of Ribbon and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. Use of Estimates and Judgments The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and judgments relied upon in preparing these consolidated financial statements include accounting for business combinations, revenue recognition for multiple element arrangements, inventory valuations, assumptions used to determine the fair value of stock-based compensation, intangible assets and goodwill valuations, legal contingencies and recoverability of Ribbon's net deferred tax assets and the related valuation allowances. Ribbon regularly assesses these estimates and records changes in estimates in the period in which they become known. Ribbon bases its estimates on historical experience and various other assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates. Reclassifications Certain reclassifications, net affecting previously reported net loss, have been made to the previously issued financial statements to conform to the current period presentation. Business Combinations The Company recognizes identifiable assets acquired and liabilities assumed at their acquisition date fair values. Goodwill as of the acquisition date is measured as the excess of consideration transferred over the net of the acquisition date fair values of the assets acquired and the liabilities assumed and represents the expected future economic benefits arising from other assets acquired in the business combination that are not individually identified and separately recognized. While the Company uses its best estimates and assumptions as part of the purchase price allocation process to accurately value assets acquired and liabilities assumed at the acquisition date, its estimates are inherently uncertain and subject to refinement. As a result, during the measurement period, which may be up to one year from the acquisition date, the Company records adjustments to the assets acquired and liabilities assumed with the corresponding offset to goodwill to the extent that it identifies adjustments to the preliminary purchase price allocation. Upon the conclusion of the measurement period or final determination of the values of assets acquired and liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the consolidated statements of operations. Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers ("ASC 606" or the "New Revenue Standard") using the modified retrospective approach. As a result, the Company changed its accounting policy for revenue recognition, which is described below and in Note 14. The Company derives revenue from two primary sources: products and services. Product revenue includes the Company's appliances and software that function together to deliver the products' essential functionality. Software and appliances are also sold on a standalone basis. Services include customer support (software updates, upgrades and technical support), consulting, design services, installation services and training. Generally, contracts with customers contain multiple performance obligations, consisting of products and services. For these contracts, the Company accounts for individual performance obligations separately if they are considered distinct. When an arrangement contains more than one performance obligation, the Company will allocate the transaction price to each performance obligation on a relative standalone selling price basis. The Company utilizes the observable price of goods and services when they are sold separately to similar customers in order to estimate standalone selling price. The Company's software licenses typically provide a perpetual right to use the Company's software. The Company also sells term-based software licenses that expire and Software-as-a-Service ("SaaS")-based software which are referred to as subscription arrangements. The Company does not customize its software nor are installation services required, as the customer has a right to utilize internal resources or a third-party service company. The software and appliances are delivered before related services are provided and are functional without professional services or customer support. The Company has concluded that its software licenses are functional intellectual property that are distinct, as the user can benefit from the software on its own. The product revenue is typically recognized upon transfer of control or when the software is made available for download, as this is the point that the user of the software can direct the use of, and obtain substantially all of the remaining benefits from, the functional intellectual property. The Company does not recognize software revenue related to the renewal of subscription software licenses earlier than the beginning of the subscription period. Appliance products are generally sold with software to provide the customer solution. The Company offers warranties on its products. Certain of the Company's warranties are considered to be assurance-type in nature and do not cover anything beyond ensuring that the product is functioning as intended. Based on the guidance in ASC 606, assurance-type warranties do not represent separate performance obligations. The Company also sells separately-priced maintenance service contracts which qualify as service-type warranties and represent separate performance obligations. The Company does not allow and has no history of accepting product returns. Services revenue includes revenue from customer support and other professional services. Customer support includes software updates on a when-and-if-available basis, telephone support, integrated web-based support and bug fixes or patches. The Company sells its customer support contracts at a percentage of list or net product price related to the support. Customer support revenue is recognized ratably over the term of the customer support agreement, which is typically one year. The Company's professional services include consulting, technical support, resident engineer services, design services and installation services. Because control transfers over time, revenue is recognized based on progress toward completion of the performance obligation. The method to measure progress toward completion requires judgment and is based on the nature of the products or services to be provided. The Company generally uses the input method to measure progress for its contracts because it believes such method best depicts the transfer of assets to the customer, which occurs as the Company incurs costs for the contracts. Under the cost-to-cost measure of progress, the progress toward completion is measured based on the ratio of costs incurred to date to the total estimated costs at completion of the performance obligation. When the measure of progress is based upon expended labor, progress toward completion is measured as the ratio of labor time expended to date versus the total estimated labor time required to complete the performance obligation. Revenue is recorded proportionally as costs are incurred or as labor is expended. Costs to fulfill these obligations include internal labor as well as subcontractor costs. Customer training includes courses offered by the Company. The related revenue is typically recognized as the training services are performed. Financial Instruments The carrying amounts of Ribbon's financial instruments approximate their fair values and include cash equivalents, investments, accounts receivable, borrowings under a revolving credit facility, accounts payable and long-term debt. All investments in marketable securities are classified as available-for-sale and are reported at fair value, with unrealized gains and losses excluded from earnings and reported, net of tax, in Accumulated other comprehensive income (loss), which is a component of stockholders' equity. Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit-related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Financial instruments with remaining maturities or that are due within one year from the balance sheet date are classified as current. Financial instruments with maturities or that are payable more than one year from the balance sheet date are classified as noncurrent. Cash and Cash Equivalents Cash equivalents are stated at fair value. Cash equivalents are liquid securities that have remaining maturities of three months or less at the date of purchase. Foreign Currency Translation For foreign subsidiaries where the functional currency is the local currency, assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Revenue and expenses are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Accumulated other comprehensive income. For foreign subsidiaries where the functional currency is the U.S. dollar, monetary assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Nonmonetary assets and liabilities are remeasured into U.S. dollars at historical exchange rates. Revenue and expense items are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Other income (expense), net. Realized and unrealized foreign currency exchange gains and losses arising from transactions denominated in currencies other than the subsidiary's functional currency are reflected in earnings. Effective on the Merger Date, the Company began to record its foreign currency gains (losses) as a component of Other income (expense), net. The Company did not reclassify amounts previously recorded within General and administrative expenses as the amounts were not material to the consolidated results of the Company. The Company recognized net foreign currency losses of $1.1 million for the year ended December 31, 2019, $4.6 million for the year ended December 31, 2018 and $0.7 million for the year ended December 31, 2017. Inventory Inventory is recorded at the lower of cost or market value using the first-in, first-out convention. The Company reduces the carrying value of inventory for those items that are potentially excess, obsolete or slow-moving based on changes in customer demand, technology developments or other economic factors. Ribbon writes down evaluation equipment at the time of shipment to its customers, as it is probable that the inventory value will not be realized. Deferred product costs represent deferred cost of revenue for product shipments to customers prior to satisfaction of Ribbon's revenue recognition criteria. The Company classifies inventory that is not expected to be consumed within one year from the balance sheet date as noncurrent and includes such inventory as a component of Other assets. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Expenditures for maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful lives of the related assets, which range from two to five years. Leasehold improvements are amortized over the lesser of the lease term or five years. When an asset is sold or retired, the cost and related accumulated depreciation or amortization are eliminated, and the resulting gain or loss, if any, is recognized in income (loss) from operations in the consolidated statement of operations. The Company reviews property and equipment for impairment in the same manner as intangible assets discussed below. Software development costs associated with internal use software are incurred in three stages of development: the preliminary project stage, the application development stage and the post-implementation stage. Costs incurred during the preliminary project and post-implementation stages are expensed as incurred. Certain qualifying costs incurred during the application development stage are capitalized as property and equipment. Internal use software is amortized on a straight-line basis over its estimated useful life of three years, beginning when the software is ready for its intended use. Intangible Assets and Goodwill Intangible assets are primarily comprised of certain intangible assets arising from the Merger and the Edgewater Acquisition. These intangible assets include a combination of in-process research and development, developed technology, customer relationships, trade names, and internal use software. Intangible assets are reviewed for impairment when events or changes in circumstances indicate that their carrying amounts may not be recoverable based upon the estimated undiscounted cash flows. Recoverability of intangible assets with estimated lives and other long-lived assets is measured by a comparison of the carrying amount of an asset or asset group to future net undiscounted cash flows expected to be generated by the asset or asset group. If these comparisons indicate that an asset is not recoverable, the Company will recognize an impairment loss for the amount by which the carrying value of the asset or asset group exceeds the related estimated fair value. Estimated fair value is based on either discounted future operating cash flows or appraised values, depending on the nature of the asset. The Company amortizes its intangible assets over their respective useful lives, with the exception of in-process research and development, which has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology, and the Company begins to amortize this asset. See Note 9 for additional information regarding the Company's intangible assets. Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. Goodwill is not amortized, but instead is tested for impairment at least annually, or more frequently if indicators of potential impairment exist, by comparing the fair value of the Company's reporting unit to its carrying value. The Company's annual testing for impairment of goodwill is completed as of November 30. The Company operates as a single operating segment with one reporting unit and consequently evaluates goodwill for impairment based on an evaluation of the fair value of the Company as a whole. Based on the results of the Company's 2019 annual impairment test, the Company determined that its carrying value exceeded its fair value. The Company performed a fair value analysis using both an Income and Market approach which encompasses a discounted cash flow analysis and a guideline public company analysis using selected multiples. The Company recorded an impairment charge in the fourth quarter of 2019 of $164.3 million. The impairment charge is reported separately in the Company's consolidated statement of operations for the year ended December 31, 2019. The Company performed its assessments for each of the years ended December 31, 2018 and 2017 and determined in each of those years that its fair value was in excess of its carrying value and accordingly, there was no impairment of goodwill. At certain times during the years ended December 31, 2019 and 2018, including at the Company's annual testing date of November 30, 2018, the Company's market capitalization was below its book value. While the Company had concluded that its fair value exceeded its carrying value at that date, the Company regularly monitored for changes in circumstances, including changes to the Company's performance, that could result in impairment of goodwill. Stock-Based Compensation The Company's stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which generally represents the vesting period, and includes an estimate of the awards that will be forfeited. The Company uses the Black-Scholes valuation model for estimating the fair value on the date of grant of stock options. The fair value of stock option awards is affected by the Company's stock price as well as valuation assumptions, including the volatility of Ribbon's stock price, expected term of the option, risk-free interest rate and expected dividends. The Company may grant performance-based stock units ("PSUs") that include a market condition to certain of its executives. The Company uses a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the volatility of each entity and the pair-wise covariance between each entity. These results are then used to calculate the grant date fair values of the PSUs. Concentrations of Credit Risk The financial instruments that potentially subject Ribbon to concentrations of credit risk are cash, cash equivalents, investments and accounts receivable. The Company's cash equivalents and investments were managed by one financial institution at December 31, 2018. Historically, the Company has not experienced significant losses due to such bank depository concentration. Certain components and software licenses from third parties used in Ribbon's products are procured from single sources of supply. The failure of a supplier, including a subcontractor, to deliver on schedule could delay or interrupt Ribbon's delivery of products and thereby materially adversely affect Ribbon's revenue and operating results. Advertising Costs Advertising costs are expensed as incurred and included as a component of Sales and marketing expense in the Company's consolidated statements of operations. Advertising expenses were $0.5 million for the year ended December 31, 2019, $0.5 million for the year ended December 31, 2018 and $0.3 million for the year ended December 31, 2017. Operating Segments The Company operates in a single segment, as the chief operating decision maker makes decisions and assesses performance at the company level. Operating segments are identified as components of an enterprise about which separate discrete financial information is utilized for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. To date, the chief operating decision maker has made such decisions and assessed performance at the company level, as one segment. The Company's chief operating decision makers are its Interim Co-Presidents and Chief Executive Officers. Loss Contingencies and Reserves Ribbon is subject to ongoing business risks arising in the ordinary course of business, including legal claims, that affect the estimation process of the carrying value of assets, the recording of liabilities and the possibility of various loss contingencies. An estimated loss contingency is accrued when it is probable that a liability has been incurred or an asset has been impaired and the amount of loss can be reasonably estimated. Ribbon regularly evaluates current information available to determine whether such amounts should be adjusted and records changes in estimates in the period they become known. An allowance for doubtful accounts is estimated based on the Company's assessment of the collectability of specific customer accounts. Ribbon accrues for royalties for technology that it licenses from vendors based on established royalty rates and usage. Ribbon is periodically contacted by third parties who claim that Ribbon's products infringe on certain intellectual property of a third party. Ribbon evaluates these claims and accrues amounts when it is probable that the obligation has been incurred and the amounts are reasonably estimable. Accounting for Leases Effective January 1, 2019, the Company adopted the new standard on accounting for leases, ASC 842, Leases ("ASC 842"). ASC 842 replaced existing lease accounting rules with a comprehensive lease measurement and recognition standard and expanded disclosure requirements (see Note 17). ASC 842 requires lessees to recognize most leases on their balance sheets and eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. The Company elected to use the alternative transition method, which allowed entities to initially apply ASC 842 at the adoption date with no subsequent adjustments to prior period lease costs for comparability. The Company elected the package of practical expedients permitted under the transition guidance, which provided that a company need not reassess whether expired or existing contracts contained a lease, the lease classification of expired or existing leases, and the amount of initial direct costs for existing leases. In connection with the adoption of ASC 842, the Company recorded additional lease assets of $43.9 million and additional lease liabilities of $47.8 million as of January 1, 2019. The difference between the additional lease assets and lease liabilities, net of the deferred tax impact, was due to the absorption of related balances into the right-of-use assets, such as deferred rent. The adoption of this standard had no impact on the Company's consolidated statements of operations or cash flows. Accounting for Income Taxes Deferred tax assets and liabilities are recognized for the expected future consequences of events that have been reflected in the consolidated financial statements. Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities and operating loss carryforwards, using tax rates expected to be in effect for the years in which the differences are expected to reverse. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company has provided for income taxes on the undistributed earnings of its non-U.S. subsidiaries as of December 31, 2019, with the exception of the Company's Irish subsidiary, as the Company does not plan to permanently reinvest these amounts outside the United States. The repatriation of the undistributed earnings would result in withholding taxes imposed on the repatriation. Consequently, the Company has recorded a tax liability of $4.8 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2019. Had the earnings of the Irish subsidiary been determined to not be permanently reinvested outside the U.S., no additional deferred tax liability would be required due to no withholding taxes or income tax expense being imposed on such repatriation. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of the benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. Defined Benefit Plans The Company has defined benefit plans for some of its employees at various international locations. The Company recognizes retirement benefit assets or liabilities in the consolidated balance sheets reflecting the funded status of pension and other retirement benefit plans. Retirement benefit assets and liabilities are adjusted for the difference between the benefit obligations and the plan assets at fair value (measured at year-end), with the offset recorded directly to stockholders' equity through accumulated other comprehensive income (loss), net of tax. The amount recorded in stockholders' equity represents the after-tax unamortized actuarial gains or losses, unamortized transition obligations and unamortized prior service costs. Recent Accounting Pronouncements The Financial Accounting Standards Board ("FASB") has issued the following accounting pronouncements, all of which became effective for the Company on January 1, 2019 and none of which had a material impact on the Company's consolidated financial statements: In July 2018, the FASB issued Accounting Standards Update ("ASU") 2018-09, Codification Improvements (“ASU 2018-09”), which contains amendments to clarify, correct errors in or make minor improvements to the FASB codification. ASU 2018-09 makes improvements to multiple topics, including but not limited to comprehensive income, debt, income taxes related to both stock-based compensation and business combinations, fair value measurement and defined contribution benefit plans. In June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of ASC 718, Compensation - Stock Compensation ("ASC 718"), to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. As a result, most of the guidance in ASC 718 associated with employee share-based payments, including most of its requirements related to classification and measurement, applies to nonemployee share-based payment arrangements. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which amends ASC 220, Income Statement - Reporting Comprehensive Income, to allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (the "Tax Act") and requires entities to provide certain disclosures regarding stranded tax effects. The Company did not elect to reclassify the income tax effects of the Tax Act from accumulated other comprehensive income to accumulated deficit. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory, which removes the prohibition in ASC 740, Income Taxes, against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. In addition, the FASB has issued the following accounting pronouncements, none of which the Company believes will have a material impact on its consolidated financial statements: In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (“ASU 2018-15”), which provides guidance on implementation costs incurred in a cloud computing arrangement (“CCA”) that is a service contract. ASU 2018-15 amends ASC 350, Intangibles - Goodwill and Other (“ASC 350”) to include in its scope implementation costs of a CCA that is a service contract and clarifies that a customer should apply the guidance in ASC 350-40 to determine which implementation costs should be capitalized in such a CCA. ASU 2018-15 was effective for the Company beginning January 1, 2020. In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20): Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans (“ASU 2018-14”), which amends ASC 715, Compensation - Retirement Benefits, to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. ASU 2018-14 was effective for the Company beginning January 1, 2020. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which changes the fair value measurement requirements of ASC 820, Fair Value Measurement. ASU 2018-13 was effective for the Company beginning January 1, 2020 for both interim and annual reporting. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"), which adds an impairment model that is based on expected losses rather than incurred losses. Under ASU 2016-13, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. In April and May 2019, the FASB issued ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments - Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments ("ASU 2019-04") and ASU 2019-05 Financial Instruments - Credit Losses (Topic 326): Targeted Transition Relief ("ASU 2019-05"), respectively. ASU 2019-04 provides transition relief for entities adopting ASU 2016-13 and ASU 2019-05 clarifies certain aspects of the accounting for credit losses, hedging activities and financial instruments in connection with the adoption of ASU 2016-13. ASU 2019-04 and ASU 2019-05 are effective with the adoption of ASU 2016-13, which was effective for the Company beginning January 1, 2020 for both interim and annual reporting periods.

NATURE OF THE BUSINESS	12 Months Ended
NATURE OF THE BUSINESS	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
NATURE OF THE BUSINESS	NATURE OF THE BUSINESS Ribbon is a leading provider of next generation ("NextGen") software solutions to telecommunications, wireless and cable service providers and enterprises of all sizes across industry verticals. With over 1,000 customers around the globe, including some of the largest telecommunications service providers and enterprises in the world, Ribbon enables service providers and enterprises to modernize their communications networks through software and provide secure RTC solutions to their customers and employees. By securing and enabling reliable and scalable IP networks, Ribbon helps service providers and enterprises adopt the next generation of software-based virtualized and cloud communications technologies to drive new, incremental revenue, while protecting their existing revenue streams. Ribbon's software solutions provide a secure way for its customers to connect and leverage multivendor, multiprotocol communications systems and applications across their networks and the cloud, around the world and in a rapidly changing ecosystem of IP-enabled devices, such as smartphones and tablets. In addition, Ribbon's software solutions secure cloud-based delivery of UC solutions - both for service providers transforming to a cloud-based network and for enterprises using cloud-based UC. Ribbon sells its software solutions through both direct sales and indirect channels globally, leveraging the assistance of resellers, and provides ongoing support to its customers through a global services team with experience in design, deployment and maintenance of some of the world's largest software IP networks.

EARNINGS (LOSS) PER SHARE

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

EARNINGS (LOSS) PER SHARE

Basic earnings (loss) per share is computed by dividing net income (loss) by the weighted average number of shares outstanding during the period. For periods in which the Company reports net income, diluted net income per share is determined by using the weighted average number of common and dilutive common equivalent shares outstanding during the period unless the effect is antidilutive.

The calculations of shares used to compute basic and diluted loss per share are as follows (in thousands):



	Year ended December 31,
	2019	2018	2017
Weighted average shares outstanding—basic	109,734	103,916	58,822
Potential dilutive common shares	—	—	—
Weighted average shares outstanding—diluted	109,734	103,916	58,822

Options to purchase the Company's common stock, unvested shares of restricted stock and unvested shares underlying performance-based stock grants aggregating 4.6 million shares for the year ended December 31, 2019 have not been included in the computation of diluted loss per share because their effect would have been antidilutive. Options to purchase the Company's common stock, unvested shares of restricted stock, unvested shares underlying performance-based stock grants and shares in connection with future purchases under the Company's Amended and Restated 2000 Employee Stock Purchase Plan, as amended (the "ESPP"), aggregating 3.1 million shares for the year ended December 31, 2018 have not been included in the computation of diluted loss per share because their effect would have been antidilutive. Options to purchase the Company's common stock, unvested shares of restricted stock and unvested shares underlying performance-based stock grants aggregating 2.5 million shares for the year ended December 31, 2017 have not been included in the computation of diluted loss per share because their effect would have been antidilutive.

BUSINESS ACQUISITONS

12 Months Ended

Dec. 31, 2019

Business Combinations [Abstract]

Business Acquisitions

BUSINESS ACQUISITIONS

Pending Merger

On November 14, 2019, Ribbon entered into an Agreement and Plan of Merger (the "ECI Merger Agreement") with Eclipse Communications Ltd., an indirect wholly-owned subsidiary of the Company ("Merger Sub"), Ribbon Communications Israel Ltd., ECI Telecom Group Ltd. ("ECI") and ECI Holding (Hungary) kft, pursuant to which Merger Sub will merge with and into ECI, with ECI surviving such merger as a wholly-owned subsidiary of Ribbon (the "ECI Merger").

The Board unanimously approved the ECI Merger Agreement and the transactions contemplated thereby. Ribbon's stockholders approved the issuance of 32.5 million shares of the Company's common stock (the "ECI Stock Consideration") as partial consideration in the ECI Merger.

As provided in the ECI Merger Agreement, at the time of the closing of the ECI Merger (the "Effective Time"), all equity securities of ECI issued and outstanding immediately prior to the Effective Time will be converted into the right to receive consideration consisting of $324 million in cash (the "ECI Cash Consideration") and ECI Stock Consideration, less the amount of indebtedness of ECI as of the Effective Time. ECI equityholders will also receive approximately $31 million from ECI's sale of real estate assets.

Anova Data, Inc.

On the Anova Acquisition Date, the Company acquired the business and technology assets of Anova, a private company headquartered in Westford, Massachusetts that provides advanced analytics solutions (the "Anova Acquisition"). The Anova Acquisition was completed in accordance with the terms and conditions of an asset purchase agreement, dated as of January 31, 2019 (the "Anova Asset Purchase Agreement"). The Company believes that the Anova Acquisition is reinforcing and extending Ribbon's strategy to expand into network optimization, security and data monetization via big data analytics and machine learning.

As consideration for the Anova Acquisition, Ribbon issued 2.9 million shares of Ribbon common stock with a fair value of $15.2 million to Anova's sellers and equity holders on the Anova Acquisition Date and held back an additional 0.3 million shares with a fair value of $1.7 million, some or all of which could be issued subject to post-closing adjustments (the "Anova Deferred Consideration"). The Anova Deferred Consideration is included as a component of Accrued expenses and other current liabilities in the Company's consolidated balance sheet at December 31, 2019.

The Anova Acquisition has been accounted for as a business combination and the financial results of Anova have been included in the Company's consolidated financial statements for the period subsequent to the Anova Acquisition Date. The results for the year ended December 31, 2019 are not significant to the Company's consolidated financial statements. The Company has not provided pro forma financial information, as the historical amounts are not significant to the Company's consolidated financial statements.

As of December 31, 2019, the valuation of acquired assets, identifiable intangible assets and certain assumed liabilities was final. The finalization of this valuation resulted in a refinement of the allocation of purchase price between the identifiable intangible assets arising from the transaction, resulting in a $2.0 million reduction to the customer relationships intangible asset and an increase of $2.0 million to the developed technology intangible asset. The purchase consideration aggregating $16.9 million has been allocated to $11.2 million of identifiable intangible assets, comprised of $5.2 million of customer relationships and $6.0 million of developed technology, and working capital items aggregating $0.2 million of net assets acquired. The remaining unallocated amount of $5.5 million has been recorded as goodwill.

The valuation of the acquired intangible assets is inherently subjective and relies on significant unobservable inputs. The Company used an income approach to value the acquired intangible assets relating to developed technology and customer relationships. The valuation for each of these intangible assets was based on estimated projections of expected cash flows to be generated by the assets, discounted to the present value at discount rates commensurate with perceived risk. The valuation assumptions take into consideration the Company's estimates of customer attrition, technology obsolescence and revenue growth projections. The Company is amortizing the identifiable intangible assets in relation to the expected cash flows from the individual intangible assets over their respective useful lives, which have a weighted average life of 6.25 years (see Note 9).

The excess of purchase consideration over net tangible and identifiable intangible assets acquired was recorded as goodwill. The goodwill is deductible for tax purposes.

Edgewater Networks, Inc.

On the Edgewater Acquisition Date, the Company completed its acquisition of Edgewater, a private company headquartered in San Jose, California. The Edgewater Acquisition was completed in accordance with the terms and conditions of the Agreement and Plan of Merger, dated as of June 24, 2018, by and among Ribbon, Merger Sub, Edgewater and Shareholder Representative Services LLC, a Colorado limited liability company, solely in its capacity as the initial holder representative (the "Edgewater Merger Agreement”).

Edgewater is a market leader in Network Edge Orchestration for the small and medium enterprise and UC market. The Company believes that the acquisition of Edgewater will allow it to offer its global customer base a complete core-to-edge product portfolio, end-to-end service assurance and analytics solutions, and a fully integrated software-defined SD-WAN service.

As consideration for the Edgewater Acquisition, Ribbon paid, in the aggregate, $46.4 million of cash, net of cash acquired, and issued 4.2 million shares of Ribbon common stock to Edgewater's selling shareholders and holders of vested in-the-money options and warrants to acquire common stock of Edgewater (the "Edgewater Selling Stakeholders") on the Edgewater Acquisition Date. Pursuant to the Edgewater Merger Agreement and subject to the terms and conditions contained therein, Ribbon agreed to pay the Edgewater Selling Stakeholders an additional $30 million of cash, $15 million of which was to be paid 6 months from the closing date and the other $15 million of which was to be paid as early as 9 months from the closing date and no later than 18 months from the closing date (the exact timing of which would depend on the amount of revenue generated from the sales of Edgewater products in 2018) ("Edgewater Deferred Consideration"). The current portion of this deferred purchase consideration was included as a component of Accrued expenses and other, and the noncurrent portion was included as a component of Other long-term liabilities in the Company's consolidated balance sheet as of December 31, 2018.

On February 15, 2019, the Company and the Edgewater Selling Stakeholders agreed to reduce the amount of Edgewater Deferred Consideration from $30 million to $21.9 million and agreed that all such deferred consideration would be payable on March 8, 2019. The Company paid the Edgewater Selling Stakeholders $21.9 million on March 8, 2019 and recorded the reduction to the Edgewater Deferred Consideration of $8.1 million in Other income (expense), net, in the Company's consolidated statement of operations and as a non-cash adjustment to reconcile net income to cash flows provided by operating activities in the Company's consolidated statement of cash flows for the year ended December 31, 2019.

The Edgewater Acquisition has been accounted for as a business combination and the financial results of Edgewater have been included in the Company's consolidated financial statements for the period subsequent to its acquisition.

As of December 31, 2019, the valuation of acquired assets, identifiable intangible assets and certain assumed liabilities was final, as the Company finalized the valuation of the assets acquired and liabilities assumed in the second quarter of 2019. A summary of the allocation of the purchase consideration for Edgewater is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Cash paid to Edgewater Selling Stakeholders	$	51,162
Less cash acquired	(4,773		)
Net cash consideration	46,389
Unpaid cash consideration	30,000
Fair value of Ribbon stock issued	30,000
Fair value of equity awards assumed (see Note 16)	747
Fair value of total consideration	$	107,136

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	16,098
Property and equipment	245
Intangible assets:
Developed technology	29,500
Customer relationships	26,100
Trade names	1,100
Goodwill	48,053
Other noncurrent assets	103
Deferred revenue	(2,749		)
Other current liabilities	(9,926		)
Deferred revenue, net of current	(669		)
Other long-term liabilities	(719		)
	$	107,136

The valuation of the acquired intangible assets is inherently subjective and relies on significant unobservable inputs. The Company used an income approach to value the acquired developed technology, customer relationships and trade name intangible assets. The valuation for each of these intangible assets was based on estimated projections of expected cash flows to be generated by the assets, discounted to the present value at discount rates commensurate with perceived risk. The valuation assumptions take into consideration the Company's estimates of customer attrition, technology obsolescence and revenue growth projections. The Company is amortizing the identifiable intangible assets arising from the Edgewater Acquisition in relation to the expected cash flows from the individual intangible assets over their respective useful lives, which have a weighted average life of 8.38 years(see Note 9). Goodwill resulting from the transaction is primarily due to expected synergies between the combined companies and is not deductible for tax purposes.

The Company's revenue for the year ended December 31, 2018 included $21.5 million of revenue and $4.3 million of net loss attributable to Edgewater since the Edgewater Acquisition Date. The Company has not provided pro forma financial information, as the historical amounts are not significant to the Company's consolidated financial statements.

GENBAND Merger

On October 27, 2017, Sonus consummated an acquisition as specified in the Merger Agreement with NewCo and GENBAND such that, following the Merger, each of Sonus and GENBAND became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. On November 28, 2017, the Company changed its name from "Sonus Networks, Inc." to "Ribbon Communications Inc."

Prior to the Merger, GENBAND was a Cayman Islands exempted company limited by shares that was formed on April 7, 2010. Through its wholly owned operating subsidiaries, GENBAND created rapid communications and applications for service providers, enterprises, independent software vendors, system integrators and developers globally. A majority of GENBAND's shares were held by JPMorgan Chase & Co. and managed by One Equity Partners ("OEP"). GENBAND shares were not listed on an exchange or quoted on any automated services, and there was no established trading market for GENBAND shares.

The Company believes that Sonus' and GENBAND's complementary products, solutions and strategies have positioned the combined company to deliver comprehensive solutions to service providers and enterprises migrating to a virtualized all-IP environment in an expanded customer and global footprint.

Pursuant to the Merger Agreement, NewCo issued 50.9 million shares of Sonus common stock to the GENBAND equity holders, with the number of shares issued in the aggregate to the GENBAND equity holders equal to the number of shares of Sonus common stock outstanding immediately prior to the closing date of the Merger, such that former stockholders of Sonus would own approximately 50%, and former shareholders of GENBAND would own approximately 50%, of the shares of NewCo common stock issued and outstanding immediately following the consummation of the Merger.

In addition, NewCo repaid GENBAND’s long-term debt, including both principal and unpaid interest, to a related party of GENBAND totaling $48.0 million and repaid GENBAND’s management fees due to an affiliate of OEP totaling $10.3 million. NewCo also issued a promissory note for $22.5 million to certain GENBAND equity holders (the "Promissory Note").

NewCo assumed the liability under GENBAND's Senior Secured Credit Agreement (the "GENBAND Credit Agreement") with Silicon Valley Bank ("SVB"), which had outstanding borrowings and letters of credit totaling $17.9 million and $2.9 million, respectively, at October 27, 2017. At October 27, 2017, the outstanding borrowings had an average interest rate of 4.67%.

The Merger has been accounted for as a business combination and the financial results of GENBAND have been included in the Company's consolidated financial statements for the period subsequent to its acquisition.

As of December 31, 2018, the valuation of acquired assets, identifiable intangible assets and certain assumed liabilities was final, as the Company finalized the valuation of the assets acquired and liabilities assumed in the third quarter of 2018. A summary of the final allocation of the purchase consideration for GENBAND is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	$	47,973
Payment of GENBAND management fees due to majority shareholder	10,302
Less cash acquired	(15,324		)
Net cash consideration	42,951
Fair value of Sonus stock issued	413,982
Promissory note issued to GENBAND equity holders	22,500
Fair value of total consideration	$	479,433

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	99,126
Property and equipment	16,770
Intangible assets:
In-process research and development	5,600
Developed technology	129,000
Customer relationships	101,300
Trade names	900
Goodwill	285,825
Other noncurrent assets	6,732
Revolving credit facility	(17,930		)
Deferred revenue	(32,390		)
Other current liabilities	(80,023		)
Deferred revenue, net of current	(6,804		)
Other long-term liabilities	(28,673		)
	$	479,433

The valuation of acquired intangible assets is inherently subjective and relies on significant unobservable inputs. The Company used an income approach to value the acquired developed technology, customer relationships and trade name intangible assets. The valuation for each of these intangible assets was based on estimated projections of expected cash flows to be generated by the assets, discounted to the present value at discount rates commensurate with perceived risk. The valuation assumptions took into consideration the Company's estimates of customer attrition, technology obsolescence and revenue growth projections. The Company will reclassify the in-process research and development intangible asset to a developed technology intangible asset in the period that the related product becomes generally available and will begin to record amortization expense for the developed technology intangible asset at that time. The Company is amortizing the identifiable intangible assets arising from the Merger in relation to the expected cash flows from the individual intangible assets over their respective useful lives, which have a weighted average life of 8.3 years (see Note 9). Goodwill resulting from the transaction is primarily due to expected synergies between the combined companies and is not deductible for tax purposes.

Pro Forma Results

The following unaudited pro forma information presents the condensed combined results of operations of Sonus and GENBAND for the year ended December 31, 2017 as if the Merger had been completed on January 1, 2016, with adjustments to give effect to pro forma events that are directly attributable to the Merger. These pro forma adjustments include a reduction of historical GENBAND revenue for the fair value adjustment related to acquired deferred revenue, an increase in amortization expense for the acquired identifiable intangible assets, a decrease in historical GENBAND interest expense reflecting the extinguishment of certain of GENBAND's debt as a result of the Merger, net of the interest expense recorded in connection with the promissory note issued to certain GENBAND equity holders as part of the purchase consideration and the elimination of revenue and costs related to sales transactions between Sonus and GENBAND. Pro forma adjustments also include the elimination of acquisition- and integration-related costs directly attributable to the acquisition and incremental stock-based compensation expense directly attributable to the acquisition from the year ended December 31, 2017.

The unaudited pro forma results do not reflect any operating efficiencies or potential cost savings that may result from the consolidation of the operations of Sonus and GENBAND. Accordingly, these unaudited pro forma results are presented for illustrative purposes and are not intended to represent or be indicative of the actual results of operations of the combined company that would have been achieved had the Merger occurred at the beginning of the periods presented, nor are they intended to represent or be indicative of future results of operations (in thousands, except per share amounts):



	Year ended December 31,
	2017
	(unaudited)
Revenue	$	615,286
Net loss	$	(69,741	)
Loss per share	$	(0.69	)

Acquisition- and Integration-Related Expenses

Acquisition- and integration-related expenses include those expenses related to acquisitions that would otherwise not have been incurred by the Company, including professional and services fees, such as legal, audit, consulting, paying agent and other fees, and expenses related to cash payments to certain former executives of the acquired businesses in connection with their employment agreements. Integration-related expenses represent incremental costs related to combining the Company and its business acquisitions, such as third-party consulting and other third-party services related to merging the previously separate companies' systems and processes.

The acquisition-related professional and services fees recorded in the year ended December 31, 2019 primarily related to the pending ECI Merger and, to a lesser extent, to the Anova Acquisition and other acquisition-related activities. The acquisition-related professional and services fees recorded in the year ended December 31, 2018 primarily related to the Merger, with nominal amounts related to the acquisition of Edgewater and other acquisition-related activities. The amounts recorded in the year ended December 31, 2017 primarily related to the Merger, with a nominal amount related to the acquisition of Taqua.

The components of acquisition- and integration-related costs incurred in the years ended December 31, 2019, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Professional and services fees (acquisition-related)	$	8,657	$	7,627	$	11,916
Management bonuses (acquisition-related)	—		1,972		931
Integration-related expenses	4,296		7,352		1,916
	$	12,953	$	16,951	$	14,763

CASH EQUIVALENTS AND INVESTMENTS

12 Months Ended

Dec. 31, 2019

Investments, All Other Investments [Abstract]

CASH EQUIVALENTS AND INVESTMENTS

The Company invests in debt instruments, primarily U.S. government-backed, municipal and corporate obligations, which management believes to be high quality (investment grade) credit instruments.

The Company did not hold any marketable securities at December 31, 2019, as its remaining available-for-sale marketable securities matured during the second quarter of 2019. In addition, the Company did not hold any cash equivalents at December 31, 2019. As a result of the Company no longer holding any marketable securities or investments at December 31, 2019, the remaining tax effect on the unrealized gain on available-for-sale marketable securities was realized in the second quarter of 2019 and is included in the income tax provision in the Company's consolidated statement of operations for the year ended December 31, 2019 as a reclassification from Unrealized gain on available-for-sale marketable securities in the Company's consolidated statement of comprehensive loss. The Company had not sold any of its marketable securities in 2019 prior to their full maturity.

During the year ended December 31, 2018, the Company sold $12.5 million of its available-for-sale securities, which it used for acquisition-related payments in connection with the Edgewater Acquisition and to support integration-related and restructuring activities in connection with the Merger. During the year ended December 31, 2017, the Company sold $51.6 million of its available-for-sale securities, primarily to provide the cash consideration and other acquisition-related payments in connection with the Merger.

During the years ended December 31, 2019 and 2018, the Company recognized nominal gross gains and losses on the sales/maturities of its marketable securities.

On a quarterly basis, the Company reviews its investments, if any, to determine if there have been any events that could create a credit impairment.

The amortized cost, gross unrealized gains and losses and fair value of the Company's cash equivalents and marketable securities at December 31, 2018 were comprised of the following (in thousands):



	December 31, 2018
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	310	$	—	$	—		$	310

Marketable securities
U.S. government agency notes	$	3,998	$	—	$	(9	)	$	3,989
Corporate debt securities	3,301		—		(6		)	3,295
	$	7,299	$	—	$	(15	)	$	7,284

Fair Value Hierarchy

Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. The three-tier fair value hierarchy is based on the level of independent, objective evidence surrounding the inputs used to measure fair value. A financial instrument's categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The fair value hierarchy is as follows:

Level 1. Level 1 applies to assets or liabilities for which there are quoted prices in active markets for identical assets or liabilities.

Level 2. Level 2 applies to assets or liabilities for which there are inputs that are directly or indirectly observable in the marketplace, such as quoted prices for similar assets or liabilities in active markets or quoted prices for identical assets or liabilities in markets with insufficient volume or infrequent transactions (less active markets).

Level 3. Level 3 applies to assets or liabilities for which there are unobservable inputs to the valuation methodology that are significant to the measurement of the fair value of the assets or liabilities.

The following table shows the fair value of the Company's financial assets at December 31, 2018. These financial assets are comprised of the Company's available-for-sale debt securities and reported under the captions Cash and cash equivalents and Marketable securities in the consolidated balance sheets (in thousands):



			Fair value measurements at December 31, 2018 using:
	Total carrying value at December 31, 2018		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	310	$	310	$	—	$	—

Marketable securities
U.S. government agency notes	$	3,989	$	—	$	3,989	$	—
Corporate debt securities	3,295		—		3,295		—
	$	7,284	$	—	$	7,284	$	—

The Company's marketable securities were valued with the assistance of valuations provided by third-party pricing services, as derived from such services' pricing models. Inputs to the models may include, but are not limited to, reported trades, executable bid and asked prices, broker/dealer quotations, prices or yields of securities with similar characteristics, benchmark curves or information pertaining to the issuer, as well as industry and economic events. The pricing services may use a matrix approach, which considers information regarding securities with similar characteristics to determine the valuation for a security. The Company is ultimately responsible for the consolidated financial statements and underlying estimates. Accordingly, the Company assesses the reasonableness of the valuations provided by the third-party pricing services by reviewing actual trade data, broker/dealer quotes and other similar data, which are obtained from quoted market prices or other sources.

ACCOUNTS RECEIVABLE, NET

12 Months Ended

Dec. 31, 2019

Receivables [Abstract]

ACCOUNTS RECEIVABLE, NET

Accounts receivable, net, consisted of the following (in thousands):



	December 31,
	2019			2018
Accounts receivable	$	193,619		$	188,522
Allowance for doubtful accounts	(913		)	(669		)
Accounts receivable, net	$	192,706		$	187,853

The Company's allowance for doubtful accounts activity was as follows (in thousands):



Year ended December 31,	Balance at beginning of year		Charges to expense		Charges (credits) to other accounts (deferred revenue)			Write-offs			Balance at end of year
2019	$	669	$	738	$	68		$	(562	)	$	913
2018	$	73	$	351	$	620		$	(375	)	$	669
2017	$	10	$	154	$	(56	)	$	(35	)	$	73

INVENTORY

12 Months Ended

Dec. 31, 2019

Inventory Disclosure [Abstract]

INVENTORY

Inventory consisted of the following (in thousands):



	December 31,
	2019			2018
On-hand final assemblies and finished goods inventories	$	13,283		$	19,879
Deferred cost of goods sold	2,441			3,798
	15,724			23,677
Less noncurrent portion (included in Other assets)	(924		)	(1,075		)
Current portion	$	14,800		$	22,602

PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT

Property and equipment consisted of the following (in thousands):



		December 31,
	Useful Life	2019			2018
Equipment	2-5 years	$	82,737		$	76,423
Software	2-5 years	27,939			24,707
Furniture and fixtures	3-5 years	1,283			1,490
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	23,975			21,220
		135,934			123,840
Less accumulated depreciation and amortization		(106,958		)	(96,798		)
Property and equipment, net		$	28,976		$	27,042

The Company recorded depreciation and amortization expense related to property and equipment of $11.9 million for the year ended December 31, 2019, $11.2 million for the year ended December 31, 2018 and $8.5 million for the year ended December 31, 2017. During each of the years ended December 31, 2019, 2018 and 2017, the Company disposed of certain property and equipment that was fully depreciated at the time of disposal, which resulted in reductions in both Cost and Accumulated depreciation.

Property and equipment under finance leases included in the amounts above were as follows (in thousands):



	December 31,
	2019			2018
Cost	$	4,401		$	2,979
Less accumulated depreciation	(1,981		)	(875		)
Property and equipment under finance leases, net	$	2,420		$	2,104

The net book values of the Company's property and equipment by geographic area were as follows (in thousands):



	December 31,
	2019		2018
United States	$	17,584	$	17,862
Canada	4,768		4,076
Asia/Pacific	5,146		3,841
Europe	1,224		1,100
Other	254		163
	$	28,976	$	27,042

INTANGIBLE ASSETS AND GOODWILL

12 Months Ended

Dec. 31, 2019

Goodwill and Intangible Assets Disclosure [Abstract]

INTANGIBLE ASSETS AND GOODWILL

The Company's intangible assets at December 31, 2019 and 2018 consisted of the following (in thousands):



December 31, 2019	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.79	188,880		100,760		88,120
Customer relationships	9.46	152,140		33,350		118,790
Trade names	5.20	2,000		1,144		856
Internal use software	3.00	730		730		—
	7.82	$	349,350	$	135,984	$	213,366



December 31, 2018	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.91	182,880		63,187		119,693
Customer relationships	9.44	146,940		22,218		124,722
Trade names	5.20	2,000		624		1,376
Internal use software	3.00	730		730		—
	7.88	$	338,150	$	86,759	$	251,391

* An in-process research and development intangible asset has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology.

Amortization expense for intangible assets for the years ended December 31, 2019, 2018 and 2017 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2019		2018		2017		Statement of operations classification
Developed technology	$	37,573	$	38,976	$	18,358	Cost of revenue - product
Customer relationships	11,132		10,203		4,145		Sales and marketing
Trade names	520		544		80		Sales and marketing
	$	49,225	$	49,723	$	22,583

In connection with the preparation of its financial statements for the fourth quarter of 2017, the Company reviewed its intangible assets and other long-lived assets for impairment indicators. The Company determined that a triggering event had occurred relative to one of its developed technology intangible assets that had been previously acquired. During 2017, the Company discontinued its ongoing development of this technology and determined that there were no alternative uses of the technology within either its existing or future product lines. As a result, the Company recorded an impairment charge of $5.5 million to write down the carrying value of the asset to zero. This expense is included as a component of Cost of revenue - product in the table above and in the Company's consolidated statement of operations for the year ended December 31, 2017.

Estimated future amortization expense for the Company's intangible assets at December 31, 2019 was as follows (in thousands):



Years ending December 31,
2020	$	48,952
2021	43,495
2022	35,092
2023	27,271
2024	20,201
Thereafter	38,355
	$	213,366

Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. The Company's annual testing for impairment of goodwill is completed as of November 30. The Company operates as a single operating segment with one reporting unit and consequently evaluates goodwill for impairment based on an evaluation of the fair value of the Company as a whole. Based on the results of the Company's 2019 annual impairment test, the Company determined that its carrying value exceeded its fair value and accordingly, the Company recorded an impairment charge of $164.3 million.

The changes in the carrying value of the Company's goodwill in the years ended December 31, 2019 and 2018 were as follows (in thousands):



	Year ended December 31,
	2019			2018
Balance at January 1
Goodwill	$	386,761		$	338,822
Accumulated impairment losses	(3,106		)	(3,106		)
	383,655			335,716
Acquisition of Anova	5,541			—
Acquisition of Edgewater	—			48,053
Write-off of goodwill attributable to dissolved subsidiary	—			(114		)
Impairment of goodwill	(164,300		)	—
Balance at December 31	$	224,896		$	383,655

The components of goodwill at December 31, 2019 and 2018 were as follows (in thousands):




	December 31,
	2019			2018
Goodwill	$	392,302		$	386,761
Accumulated impairment losses	(167,406		)	(3,106		)
	$	224,896		$	383,655

ACCRUED EXPENSES AND OTHER

12 Months Ended

Dec. 31, 2019

Payables and Accruals [Abstract]

ACCRUED EXPENSES AND OTHER

Accrued expenses and other consisted of the following (in thousands):



	December 31,
	2019		2018
Employee compensation and related costs	$	27,166	$	42,852
Professional fees	13,331		7,994
Deferred purchase consideration - business acquisitions	1,700		15,000
Other	14,503		18,417
	$	56,700	$	84,263

RESTRUCTURING AND FACILITIES CONSOLIDATION INTIATIVES

12 Months Ended

Dec. 31, 2019

Restructuring and Related Activities [Abstract]

RESTRUCTURING AND FACILITIES CONSOLIDATION INITIATIVES

The Company recorded restructuring and related expense aggregating $16.4 million in the year ended December 31, 2019, $17.0 million in the year ended December 31, 2018 and $9.4 million in the year ended December 31, 2017. Restructuring and related expense includes restructuring expense (primarily severance and related costs), estimated future variable lease costs for vacated properties with no intent or ability of sublease, and accelerated rent amortization expense.

For restructuring events that involve lease assets and liabilities, the Company applies lease reassessment and modification guidance and evaluates the right-of-use assets for potential impairment. If the Company plans to exit all or distinct portions of a facility and does not have the ability or intent to sublease, the Company will accelerate the amortization of each of those lease components through the vacate date. The accelerated amortization is recorded as a component of Restructuring and related expense in the Company's consolidated statements of operations. Related variable lease expenses will continue to be expensed as incurred through the vacate date, at which time the Company will reassess the liability balance to ensure it appropriately reflects the remaining liability associated with the premises and record a liability for the estimated future variable lease costs.

The components of restructuring and related expense for the year ended December 31, 2019 were as follows (in thousands):



Severance and related costs	$	11,179
Variable and other facilities-related costs	1,528
Accelerated amortization of lease assets due to cease-use	3,692
	$	16,399

Prior to the adoption of ASC 842, the Company recorded restructuring accruals for future lease obligations related to vacated facilities at the time that it ceased usage of the respective facility. The components of Restructuring and related expense recorded in the years ended December 31, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2018		2017
Severance and related costs	15,217		8,925
Facilities	1,798		511
	$	17,015	$	9,436

2019 Restructuring and Facilities Consolidation Initiative

In June 2019, the Company implemented a restructuring plan to further streamline the Company's global footprint, improve its operations and enhance its customer delivery (the "2019 Restructuring Initiative"). The 2019 Restructuring Initiative includes facility consolidations, refinement of the Company's research and development activities, and a reduction in workforce. In connection with this initiative, the Company expects to reduce its focus on hardware and appliance-based development over time and to increase its development focus on software virtualization, functional simplicity and important customer requirements. The facility consolidations under the 2019 Restructuring Initiative (the "Facilities Initiative") include a consolidation of the Company's North Texas sites into a single campus, housing engineering, customer training and support, and administrative functions, as well as a reduction or elimination of certain excess and duplicative facilities worldwide. In addition, the Company intends to substantially consolidate its global software laboratories and server farms into two lower cost North American sites. The Company continues to evaluate its properties included in the Facilities Initiative for accelerated amortization and/or right-of-use asset impairment. The Company expects that the actions under the Facilities Initiative will be completed by the end of 2020.

In connection with the 2019 Restructuring Initiative, the Company recorded restructuring and related expense of $11.2 million in the year ended December 31, 2019, comprised of $6.1 million for severance and related costs for approximately 120 employees, $1.4 million for variable and other facilities-related costs and $3.7 million for accelerated amortization of lease assets. The Company expects that all of the amount accrued for severance and related costs will be paid in 2020. The Company estimates that it will record nominal, if any, additional restructuring and related expense related to severance and related costs under the 2019 Restructuring Initiative.

Accelerated amortization of lease assets is recognized from the date that the Company commences the plan to fully or partially vacate a facility, for which there is no intent or ability to enter into a sublease, through the final vacate date. The $3.7 million of accelerated rent amortization recorded in 2019 that is included as a component of restructuring and related expense is not included in the table below, as the liability for the total lease payments for each respective facility is included as a component of Operating lease liabilities in the Company's consolidated balance sheet at December 31, 2019, both current and noncurrent (see Note 16). The Company may incur additional future expense if it is unable to sublease other locations included in the Facilities Initiative.

A summary of the 2019 Restructuring Initiative accrual activity, excluding the accelerated amortization of lease assets, for the year ended December 31, 2019 is as follows (in thousands):



	Balance at January 1, 2019		Initiatives charged to expense		Cash payments			Balance at December 31, 2019
Severance	$	—	$	6,103	$	(3,993	)	2,110
Facilities	—		1,372		(381		)	991
	$	—	$	7,475	$	(4,374	)	$	3,101

Merger Restructuring Initiative

In connection with the Merger, the Company's management approved a restructuring plan in the fourth quarter of 2017 to eliminate certain redundant positions and facilities within the combined companies (the "Merger Restructuring Initiative"). In connection with this initiative, the Company recorded $5.2 million of restructuring and related expense in 2019, virtually all of which was for severance and related costs for approximately 40 employees. The Company recorded $16.1 million of restructuring and related expense in 2018 in connection with this initiative, comprised of $14.7 million for severance for approximately 275 additional employees and $1.4 million for redundant facilities in the Czech Republic, Canada and the U.S., and $8.5 million of restructuring and related expense in 2017 for severance and related costs for approximately 120 employees. The Merger Restructuring Initiative is substantially complete, and the Company anticipates it will record nominal future expense, if any, in connection with this initiative. In connection with the adoption of ASC 842, which was effective on January 1, 2019, the Company wrote off the remaining restructuring accrual related to facilities. The Company expects that the amount accrued at December 31, 2019 for severance will be paid by the end of the first half of 2020.

Summaries of the Merger Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Initiatives charged to expense		Adjustment for the impact of ASC 842 adoption			Cash payments			Balance at December 31, 2019
Severance	$	1,910	$	5,076	$	—		$	(6,577	)	$	409
Facilities	771		156		(771		)	(156		)	—
	$	2,681	$	5,232	$	(771	)	$	(6,733	)	$	409



Year ended December 31, 2018	Balance at January 1, 2018		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2018
Severance	$	7,595	$	14,735	$	(5	)	$	(20,415	)	$	1,910
Facilities	—		1,399		—			(628		)	771
	$	7,595	$	16,134	$	(5	)	$	(21,043	)	$	2,681

Assumed Restructuring Initiative

The Company assumed GENBAND's previously recorded restructuring liability, totaling $4.1 million, on the Merger Date (the "GENBAND Restructuring Initiative"). Of this amount, $3.7 million related to severance and related costs and $0.4 million related to facilities. Subsequent to the Merger, the Company recorded $0.6 million in the aggregate in connection with the GENBAND Restructuring Initiative, comprised of $0.9 million of restructuring and related expense in 2018 and a credit of $0.3 million to restructuring and related expense in 2017 for changes in estimated costs for this previously recorded and assumed restructuring initiative, primarily changes in negotiated severance to employees in certain international locations and changes in estimated sublease income for restructured facilities. The GENBAND Restructuring Initiative is complete, and the Company does not expect to record future expense in connection with this initiative. In connection with the adoption of ASC 842, which was effective on January 1, 2019, the Company wrote off the remaining restructuring accrual related to facilities.

Summaries of the GENBAND Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Adjustment for the impact of ASC 842 adoption			Balance at December 31, 2019
Facilities	$	117	$	(117	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2018
Severance	$	1,916	$	487	$	(2,403	)	$	—
Facilities	205		399		(487		)	117
	$	2,121	$	886	$	(2,890	)	$	117

2016 Restructuring Initiative

In July 2016, the Company announced a program (the "2016 Restructuring Initiative") to further accelerate its investment in new technologies to address the communications industry's migration to a cloud-based architecture and to support the Company's strategic initiatives, such as new products and an expanded go-to-market footprint in selected geographies and discrete vertical markets. The Company recorded $2.0 million of restructuring and related expense in the aggregate in connection with this initiative, comprised of $1.9 million for severance and related costs and $0.1 million to abandon its facility in Rochester, New York (the "Rochester Facility"). The actions under the 2016 Restructuring Initiative were completed in 2019, and accordingly, no additional expense will be recorded in connection with this initiative.

In connection with the 2016 Restructuring Initiative, the Company recorded $0.5 million of restructuring expense in the year ended December 31, 2017, including adjustments for changes in estimated costs, comprised of $0.4 million for severance and related costs and $0.1 million related to the Company's Rochester Facility.

Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Cash payments			Balance at December 31, 2019
Facilities	$	58	$	(58	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Cash payments			Balance at December 31, 2018
Facilities	$	95	$	(37	)	$	58

Taqua Restructuring Initiative

In connection with the acquisition of Taqua, the Company's management approved a restructuring plan in the third quarter of 2016 to eliminate certain redundant positions within the combined companies. On October 24, 2016, the Audit Committee of the Board of Directors of the Company approved a broader Taqua restructuring plan related to headcount and redundant facilities (both restructuring plans, the "Taqua Restructuring Initiative"). The Company recorded $1.8 million of restructuring and related expense in the aggregate in connection with this initiative, comprised of $1.2 million for severance and related costs and $0.6 million related to the elimination of redundant facilities, including adjustments recorded for changes in cost estimates for the planned restructuring activities. Of this amount, $0.7 million was recorded in 2017, comprised of $0.2 million for severance and related costs and $0.5 million related to redundant facilities. The actions under the Taqua Restructuring Initiative have been completed and accordingly, no additional expense will be recorded in connection with this initiative. In connection with the adoption of ASC 842, which was effective on January 1, 2019, the Company wrote off the remaining restructuring accrual related to facilities.

Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Adjustment for the impact of ASC 842 adoption			Balance at December 31, 2019
Facilities	$	33	$	(33	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Cash payments			Balance at December 31, 2018
Facilities	$	365	$	(332	)	$	33

Balance Sheet Classification

The current portions of accrued restructuring are included as a component of Accrued expenses in the consolidated balance sheets. The long-term portions of accrued restructuring are included as a component of Other long-term liabilities in the consolidated balance sheets. The long-term portions of accrued restructuring were $0.9 million at December 31, 2019 and $0.5 million at December 31, 2018. The amount recorded as long-term at December 31, 2018 represented future lease payments on restructured facilities.

DEBT	12 Months Ended
DEBT	Dec. 31, 2019
Debt Disclosure [Abstract]
DEBT	DEBT Assumed Senior Secured Credit Agreement On the Merger Date and in connection with the Merger, the Company assumed the GENBAND Credit Agreement with SVB, which had outstanding borrowings and letters of credit totaling $17.9 million and $2.9 million, respectively, and an average interest rate of 4.67%. GENBAND had entered into the GENBAND Credit Agreement with SVB effective July 1, 2016, with two of its operating subsidiaries as borrowers and GENBAND as the guarantor. The GENBAND Credit Agreement had a maturity date of July 1, 2019 and provided for revolving loans, including letters of credit and swingline loans, not to exceed $50 million in total, with potential further increases of $75 million available for a total revolving line of credit of up to $125 million. The GENBAND Credit Agreement was superseded by a Senior Secured Credit Facilities Credit Agreement (the "2017 Credit Facility"), which was entered into on December 21, 2017 and is discussed below. Senior Secured Credit Facility On December 21, 2017, the Company entered into the 2017 Credit Facility by and among the Company, as a guarantor, Sonus Networks, Inc., as the borrower (“Borrower”), SVB, as administrative agent (in such capacity, the “Administrative Agent”), issuing lender, swingline lender and lead arranger and the lenders party thereto (each referred to individually as a “Lender”, and collectively, the “Lenders”), which refinanced the GENBAND Credit Agreement. The 2017 Credit Facility included $100 million of commitments, the full amount of which was available for revolving loans, a $15 million sublimit that was available for letters of credit and a $15 million sublimit that was available for swingline loans. The 2017 Credit Facility contained procedures for additional financial institutions to become lenders or for any existing lender to increase its commitment under the facility, subject to an available increase of $50 million for all incremental commitments under the 2017 Credit Facility. On June 24, 2018, the Company amended the 2017 Credit Facility (the "2018 Credit Facility") to, among other things, permit the Edgewater Acquisition and related transactions. The indebtedness and other obligations under the 2018 Credit Facility were unconditionally guaranteed on a senior secured basis by the Company and each other material U.S. domestic subsidiary of the Company (collectively, the "Guarantors"). The 2018 Credit Facility was secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including the Company. The 2018 Credit Facility required periodic interest payments on outstanding borrowings until maturity. The Borrower could prepay all revolving loans under the 2018 Credit Facility at any time without premium or penalty (other than customary LIBOR breakage costs), subject to certain notice requirements. Revolving loans under the 2018 Credit Facility bore interest at the Borrower’s option at either the Eurodollar (LIBOR) rate plus a margin ranging from 2.50% to 3.00% per year or the base rate (the highest of the Federal Funds rate plus 0.50%, or the prime rate announced from time to time in The Wall Street Journal) plus a margin ranging from 1.50% to 2.00% per year (such margins being referred to as the “Applicable Margin”). The Applicable Margin varied depending on the Company’s consolidated leverage ratio (as defined in the 2018 Credit Facility). The base rate and the LIBOR rate were each subject to a zero percent floor. The Borrower was charged a commitment fee ranging from 0.25% to 0.40% per year on the daily amount of the unused portions of the commitments under the 2018 Credit Facility. Additionally, with respect to all letters of credit outstanding under the 2018 Credit Facility, the Borrower was charged a fronting fee of 0.125% per year and an outstanding letter of credit fee equal to the Applicable Margin for base rate loans ranging from 1.50% to 2.00% times the amount of the outstanding letters of credit. The 2018 Credit Facility required compliance with certain financial covenants, including a minimum consolidated quick ratio, minimum consolidated interest coverage ratio and maximum consolidated leverage ratio, all of which were defined in the 2018 Credit Facility and tested on a quarterly basis. In addition, the 2018 Credit Facility contained various covenants that, among other restrictions, limited the Company’s and its subsidiaries’ ability to enter into certain types of transactions, including, but not limited to: incurring or assuming indebtedness, making acquisitions or engaging in mergers, making investments, repurchasing equity and paying dividends, selling or otherwise transferring assets, changing the nature of its business and amending or making prepayments on certain junior debt. The Company was in compliance with all covenants of the 2018 Credit Facility as of December 31, 2018. The 2018 Credit Facility contained events of default that are customary for a secured credit facility. If an event of default relating to bankruptcy or other insolvency events with respect to a borrower occurred, all obligations under the 2018 Credit Facility would immediately become due and payable. If any other event of default existed under the 2018 Credit Facility, the lenders could accelerate the maturity of the obligations outstanding under the 2018 Credit Facility and exercise other rights and remedies, including charging a default rate of interest equal to 2.00% per year above the rate that would otherwise be applicable. In addition, if any event of default existed under the 2018 Credit Facility, the lenders could commence foreclosure or other actions against the collateral. If any default existed under the 2018 Credit Facility, or if the Borrower was unable to make any of the representations and warranties as stated in the 2018 Credit Facility at the applicable time, the Borrower would be unable to borrow funds or have letters of credit issued under the 2018 Credit Facility, which, depending on the circumstances prevailing at that time, could have a material adverse effect on the Borrower’s liquidity and working capital. At December 31, 2018, the Company had an outstanding debt balance of $55.0 million at a weighted average interest rate of 5.96% and $2.7 million of outstanding letters of credit at an interest rate of 1.75% under the 2018 Credit Facility. The 2018 Credit Facility was superseded by a Senior Secured Credit Facilities Credit Agreement (the "2019 Credit Facility") which was entered into on April 29, 2019 and which is discussed below. 2019 Credit Facility On April 29, 2019, the Company, as guarantor, and Ribbon Communications Operating Company, Inc., as borrower, entered into the 2019 Credit Facility, which provides for a $50 million term loan facility that was advanced in full on April 29, 2019 and a $100 million revolving line of credit. The 2019 Credit Facility also includes procedures for additional financial institutions to become syndicate lenders, or for any existing lender to increase its commitment under either the term loan facility or the revolving loan facility, subject to an aggregate increase of $75 million for incremental commitments under the 2019 Credit Facility. The 2019 Credit Facility is scheduled to mature in April 2024. At December 31, 2019, the Company had an outstanding term loan debt balance of $48.8 million at an interest rate and an outstanding revolving line of credit balance of $8.0 million with a combined average interest rate of 3.30%, and $5.4 million of outstanding letters of credit at an interest rate of 1.50%. The indebtedness and other obligations under the 2019 Credit Facility are unconditionally guaranteed on a senior secured basis by the Company and each other material U.S. domestic subsidiary of the Company (collectively, the “Guarantors”). The 2019 Credit Facility is secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including the Company. The 2019 Credit Facility requires periodic interest payments on any outstanding borrowings under the facility. The Borrower may prepay all revolving loans under the 2019 Credit Facility at any time without premium or penalty (other than customary LIBOR breakage costs), subject to certain notice requirements. Revolving loans under the 2019 Credit Facility bear interest at the Borrower’s option at either the Eurodollar (LIBOR) rate plus a margin ranging from 1.50% to 3.00% per year or the base rate (the highest of the Federal Funds rate plus 0.50%, or the prime rate announced from time to time in The Wall Street Journal) plus a margin ranging from 0.50% to 2.00% per year (such margins being referred to as the “Applicable Margin”). The Applicable Margin varies depending on the Company’s consolidated leverage ratio (as defined in the 2019 Credit Facility). The base rate and the LIBOR rate are each subject to a zero percent floor. The 2019 Credit Facility requires compliance with certain financial covenants, including a minimum consolidated quick ratio, minimum consolidated fixed charge coverage ratio and maximum consolidated leverage ratio, all of which are defined in the 2019 Credit Facility and tested on a quarterly basis. The Company was in compliance with all covenants of the 2019 Credit Facility at December 31, 2019. In addition, the 2019 Credit Facility contains various covenants that, among other restrictions, limit the Company’s and its subsidiaries’ ability to enter into certain types of transactions, including, but not limited to: incurring or assuming indebtedness; granting or assuming liens; making acquisitions or engaging in mergers; making dividend and certain other restricted payments; making investments; selling or otherwise transferring assets; engaging in transactions with affiliates; entering into sale and leaseback transactions; entering into burdensome agreements; changing the nature of its business; modifying its organizational documents; and amending or making prepayments on certain junior debt. The 2019 Credit Facility contains events of default that are customary for a secured credit facility. If an event of default relating to bankruptcy or other insolvency events with respect to a borrower occurs, all obligations under the 2019 Credit Facility will immediately become due and payable. If any other event of default exists under the 2019 Credit Facility, the lenders may accelerate the maturity of the obligations outstanding under the 2019 Credit Facility and exercise other rights and remedies, including charging a default rate of interest equal to 2.00% per year above the rate that would otherwise be applicable. In addition, if any event of default exists under the 2019 Credit Facility, the lenders may commence foreclosure or other actions against the collateral. If any default exists under the 2019 Credit Facility, or if the Borrower is unable to make any of the representations and warranties as stated in the 2019 Credit Facility at the applicable time, the Borrower will be unable to borrow funds or have letters of credit issued under the 2019 Credit Facility, which, depending on the circumstances prevailing at that time, could have a material adverse effect on the Borrower’s liquidity and working capital. Promissory Note In connection with the Merger, on October 27, 2017, the Company issued a promissory note for $22.5 million to certain of GENBAND's equityholders (the "Promissory Note"). The Promissory Note did not amortize and the principal thereon was payable in full on the third anniversary of its execution. Interest on the Promissory Note was payable quarterly in arrears and accrued at a rate of 7.5% per year for the first six months after issuance, and thereafter at a rate of 10% per year. The failure to make any payment under the Promissory Note when due and, with respect to payment of any interest, the continuation of such failure for a period of thirty days thereafter, constituted an event of default under the Promissory Note. If an event of default occurred under the Promissory Note, the payees could declare the entire balance of the Promissory Note due and payable (including principal and accrued and unpaid interest) within five business days of the payees' notification to the Company of such acceleration. Interest that was not paid on the interest payment date would increase the principal amount of the Promissory Note. At December 31, 2018, the Promissory Note balance was $24.1 million, comprised of $22.5 million of principal, plus $1.6 million of interest converted to principal. On April 29, 2019, concurrently with the closing of the 2019 Credit Facility as discussed above, the Company repaid in full all outstanding amounts under the Promissory Note, aggregating $24.7 million. The Company did not incur any early termination penalties in connection with this repayment.

LONG-TERM LIABILITIES

12 Months Ended

Dec. 31, 2019

Other Liabilities [Abstract]

LONG-TERM LIABILITIES

Long-term liabilities consisted of the following (in thousands):



	December 31,
	2019			2018
Finance lease obligations	$	3,149		$	2,363
Deferred rent	—			3,039
Restructuring	3,510			979
Pension obligations	9,954			7,006
Taxes payable	1,991			1,818
Deferred purchase consideration	—			30,000
Other	1,683			2,425
	20,287			47,630
Current portion	(3,698		)	(16,833		)
Long-term liabilities, net of current portion	$	16,589		$	30,797

The current portions of long-term liabilities are included as components of Accrued expenses and other in the Company's consolidated balance sheets.

REVENUE RECOGNITION

12 Months Ended

Dec. 31, 2019

Revenue from Contract with Customer [Abstract]

REVENUE RECOGNITION

Effective January 1, 2018, the Company adopted the New Revenue Standard using the modified retrospective option and identified the necessary changes to its policies, processes, systems and controls. Under the modified retrospective method, the Company is applying the New Revenue Standard to all contracts not yet completed as of January 1, 2018, recognizing in beginning Accumulated deficit an adjustment for the cumulative effect of the change and providing additional disclosures comparing results to those as if the Company was still following the previous accounting standards. Under ASC 605, Revenue Recognition ("ASC 605"), the Company concluded it did not have vendor-specific objective evidence of selling price ("VSOE") for certain elements in software bundled arrangements, which resulted in revenue being recognized ratably over the longest performance period. Additionally, under ASC 606 for arrangements with certain customers that include acceptance criteria, revenue is recognized when the customer obtains control, as the Company believes acceptance is perfunctory. Under ASC 605, revenue was deferred until acceptance was received. The Company is also capitalizing incremental commission fees as a result of obtaining contracts and will amortize the asset based on the transfer of services to which the asset relates, which is approximately five years. The cumulative effect of capitalizing commission fees was not material at January 1, 2018. In connection with the adoption of ASC 606, as of January 1, 2018, the Company recorded an adjustment to decrease Accumulated deficit by $12.2 million (net of tax, which was $0 due to the full valuation allowance).

The Company's typical performance obligations include the following:



Performance Obligation	When Performance Obligation is Typically Satisfied	When Payment is Typically Due
Software and Product Revenue
Software licenses (perpetual or term)	Upon transfer of control; typically, when made available for download (point in time)	Generally, within 30 days of invoicing except for term licenses, which may be paid for over time

Software licenses (subscription)	Upon activation of hosted site (over time)	Generally, within 30 days of invoicing

Appliances	When control of the appliance passes to the customer; typically, upon delivery (point in time)	Generally, within 30 days of invoicing

Software upgrades	Upon transfer of control; typically, when made available for download (point in time)	Generally, within 30 days of invoicing

Customer Support Revenue
Customer support	Ratably over the course of the support contract (over time)	Generally, within 30 days of invoicing

Professional Services
Other professional services (excluding training services)	As work is performed (over time)	Generally, within 30 days of invoicing (upon completion of services)

Training	When the class is taught (point in time)	Generally, within 30 days of services being performed

Significant Judgments

The Company's contracts with customers often include promises to transfer multiple products and services to the customer. Determining whether products and services are considered distinct performance obligations that should be accounted for separately versus together may require significant judgment.

Judgment is required to determine the standalone selling price for each distinct performance obligation. The Company typically has more than one standalone selling price ("SSP") for individual products and services due to the stratification of those products and services by customers and circumstances. In these instances, the Company may use information such as the size of the customer and geographic region in determining the SSP.

Deferred Revenue

Deferred revenue is a contract liability representing amounts collected from or invoiced to customers in excess of revenue recognized. This results primarily from the billing of annual customer support agreements where the revenue is recognized over the term of the agreement. The value of deferred revenue will increase or decrease based on the timing of invoices and recognition of revenue.

Disaggregation of Revenue

The Company disaggregates its revenue from contracts with customers based on the nature of the products and services and the geographic regions in which each customer is domiciled.

The Company's total revenue for the years ended December 31, 2019, 2018 and 2017 was disaggregated geographically as follows:



Year ended December 31, 2019	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	170,937	$	133,271	$	37,085	$	341,293
Europe, Middle East and Africa	42,262		43,186		12,279		97,727
Japan	13,065		11,692		5,842		30,599
Other Asia Pacific	17,552		16,106		4,879		38,537
Other	18,214		29,973		6,768		54,955
	$	262,030	$	234,228	$	66,853	$	563,111



Year ended December 31, 2018	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	169,510	$	132,282	$	35,832	$	337,624
Europe, Middle East and Africa	37,833		46,856		11,794		96,483
Japan	23,108		11,234		5,069		39,411
Other Asia Pacific	30,575		12,321		4,358		47,254
Other	17,988		31,273		7,872		57,133
	$	279,014	$	233,966	$	64,925	$	577,905



Year ended December 31, 2017	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	121,121	$	75,040	$	22,896	$	219,057
Europe, Middle East and Africa	23,352		17,471		3,742		44,565
Japan	10,252		10,282		3,855		24,389
Other Asia Pacific	14,693		5,901		1,952		22,546
Other	11,701		6,041		1,643		19,385
	$	181,119	$	114,735	$	34,088	$	329,942

The Company's product revenue from its direct sales program and from indirect sales through its channel partner program for the years ended December 31, 2019, 2018 and 2017 was as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Indirect sales through channel program	$	94,639	$	69,232	$	43,138
Direct sales	167,391		209,782		137,981
	$	262,030	$	279,014	$	181,119

The Company's product revenue from sales to enterprise customers and from sales to service provider customers for the years ended December 31, 2019, 2018 and 2017 was as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Sales to enterprise customers	$	70,548	$	57,534	$	35,592
Sales to service provider customers	191,482		221,480		145,527
	$	262,030	$	279,014	$	181,119

Revenue Contract Balances

The timing of revenue recognition, billings and cash collections results in billed accounts receivable, unbilled receivables, which are contract assets, and customer advances and deposits, which are contract liabilities, in the Company's consolidated balance sheets. Amounts are billed as work progresses in accordance with agreed-upon contractual terms, either at periodic intervals or upon achievement of contractual milestones. Completion of services and billing may occur subsequent to revenue recognition, resulting in contract assets. The Company may receive advances or deposits from its customers before revenue is recognized, resulting in contract liabilities which are classified as deferred revenue. These assets and liabilities are reported in the Company's consolidated balance sheets on a contract-by-contract basis as of the end of each reporting period. Changes in the contract asset and liability balances during the years ended December 31, 2019 and 2018 were not materially impacted by any factors other than billing and revenue recognition. Nearly all of the Company's deferred revenue balance is related to services revenue, primarily customer support contracts. Unbilled receivables stem primarily from engagements where services have been performed; however, billing cannot occur until services are completed.

In some arrangements, the Company allows customers to pay for term-based software licenses and products over the term of the software license. The Company also sells SaaS-based software under subscription arrangements, with payment terms over the term of the SaaS agreement. Amounts recognized as revenue in excess of amounts billed are recorded as unbilled receivables. Unbilled receivables that are anticipated to be invoiced in the next twelve months are included in Accounts receivable on the Company's consolidated balance sheets. The changes in the Company's accounts receivable, unbilled receivables and deferred revenue balances for the years ended December 31, 2019 and 2018 were as follows (in thousands):



	Accounts receivable			Unbilled accounts receivable		Deferred revenue (current)			Deferred revenue (long-term)
Balance at January 1, 2019	$	174,310		$	13,543	$	105,087		$	17,572
Increase (decrease), net	(5,808		)	10,661		(4,681		)	2,910
Balance at December 31, 2019	$	168,502		$	24,204	$	100,406		$	20,482



	Accounts receivable		Unbilled accounts receivable			Deferred revenue (current)		Deferred revenue (long-term)
Balance at January 1, 2018	$	149,122	$	16,034		$	100,571	$	14,184
Increase (decrease), net	25,188		(2,491		)	4,516		3,388
Balance at December 31, 2018	$	174,310	$	13,543		$	105,087	$	17,572

The Company recognized approximately $94 million of revenue in the year ended December 31, 2019 that was recorded as deferred revenue at December 31, 2018 and approximately $84 million of revenue in the year ended December 31, 2018 that was recorded as deferred revenue at December 31, 2017. Of the Company's deferred revenue reported as long-term in its consolidated balance sheet at December 31, 2019, the Company expects that approximately $13 million will be recognized as revenue in 2021, approximately $4 million will be recognized as revenue in 2022 and approximately $3 million will be recognized as revenue in 2023 and beyond.

All freight-related customer invoicing is recorded as revenue, while the shipping and handling costs that occur after control of the promised goods or services transfer to the customer are reported as fulfillment costs, a component of Cost of revenue - product in the Company's consolidated statements of operations.

Deferred Commissions Cost

Sales commissions earned by the Company's employees are considered incremental and recoverable costs of obtaining a contract with a customer. Under ASC 605, the costs associated with obtaining a customer contract were expensed in the period the revenue was earned. Under ASC 606, these payments have been deferred on our consolidated balance sheet and are being amortized over the expected life of the customer contract, which is five years. At December 31, 2019 and 2018, the Company had $3.6 million and $2.7 million, respectively, of deferred sales commissions capitalized.

Adoption of ASC 606

Under the modified retrospective method, the Company applied ASC 606 to those contracts that were not completed as of January 1, 2018. Results for reporting periods beginning January 1, 2018 are presented under ASC 606, and prior period amounts have not been adjusted and are reported in accordance with the Company's historical accounting treatment under ASC 605.

The Company recorded a net reduction to Accumulated deficit of $12.2 million (net of tax, which was $0 due to the full valuation allowance) at January 1, 2018 due to the cumulative impact of adopting ASC 606. Had the Company continued to recognize revenue under ASC 605, the Company would have recognized approximately $16 million less revenue in the year ended December 31, 2018. Incremental costs that would have been recognized had the Company continued to recognize revenue under ASC 605 would not have been material to the Company's consolidated results of operations.

COMMON STOCK REPURCHASES	12 Months Ended
COMMON STOCK REPURCHASES	Dec. 31, 2019
Equity [Abstract]
COMMON STOCK REPURCHASES	COMMON STOCK REPURCHASES In the second quarter of 2019, the Company's Board of Directors (the "Board") approved a stock repurchase program (the "Repurchase Program") pursuant to which the Company may repurchase up to $75 million of its common stock prior to April 18, 2021. Repurchases under the Repurchase Program may be made in the open market, in privately negotiated transactions or otherwise, with the amount and timing of repurchases depending on market conditions and corporate discretion. The Repurchase Program does not obligate the Company to acquire any particular amount of common stock and may be extended, modified, suspended or discontinued at any time at the Board's discretion. The stock repurchases are being funded using the Company's working capital. During the year ended December 31, 2019, the Company spent $4.5 million, including transaction fees, to repurchase and retire 1.0 million shares of its common stock under the Repurchase Program. At December 31, 2019, the Company had $70.5 million remaining under the Repurchase Program for future repurchases. The Company had previously implemented a stock repurchase program (the "Sonus Repurchase Program"), which Ribbon did not assume in connection with the Merger. The Company did not repurchase any shares under the Sonus Repurchase Program during the year ended December 31, 2017.

STOCK-BASED COMPENSATION PLANS

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

STOCK-BASED COMPENSATION PLANS

2019 Stock Incentive Plan

At the Company's annual meeting of stockholders held on June 5, 2019, the Company's stockholders approved the Ribbon Communications Inc. Incentive Award Plan (the "2019 Plan"). The 2019 Plan had previously been approved by the Board, subject to stockholder approval. Under the 2019 Plan, the Company may grant awards up to 7.0 million shares of common stock (subject to adjustment in the event of stock splits and other similar events), plus 5.1 million shares of common stock that remained available for issuance under the Company's Amended and Restated Stock Incentive Plan (the "2007 Plan") on June 5, 2019, plus any shares covered by awards under the 2007 Plan (or the Company's other prior equity compensation plans) that again become available for grant pursuant to the provisions of the 2007 Plan. The 2019 Plan provides for the grant of options to purchase the Company's common stock ("stock options"), stock appreciation rights ("SARs"), restricted stock awards ("RSAs"), performance-based stock awards ("PSAs"), restricted stock units ("RSUs"), performance-based stock units ("PSUs") and other stock- or cash-based awards. Awards can be granted under the 2019 Plan to the Company's employees, officers and non-employee directors, as well as consultants and advisors of the Company and its subsidiaries. At December 31, 2019, there were 7.1 million shares available for future issuance under the 2019 Plan.

2007 Plan

The Company's 2007 Plan provides for the award of stock options, SARs, RSAs, RSUs, PSAs, PSUs and other stock-based awards to employees, officers, non-employee directors, consultants and advisors of the Company and its subsidiaries. On and following June 5, 2019, with the exception of shares underlying awards outstanding as of that date, no additional shares may be granted under the 2007 Plan.

2002 Stock Option Plan

In connection with the Edgewater Acquisition, the Company assumed Edgewater's Amended and Restated 2002 Stock Option Plan (the "Edgewater Plan") for all outstanding options as of the Edgewater Acquisition Date (the "Edgewater Options"). The Edgewater Options were converted to Ribbon stock options (the "Ribbon Replacement Options") using a conversion factor of 0.17, which was calculated based on the acquisition consideration of $1.20 per share of Edgewater common stock divided by the weighted average of the closing price of Ribbon common stock for the ten consecutive days ending with the trading day that preceded the Edgewater Acquisition Date. This conversion factor was also used to convert the exercise prices of the Edgewater Options to Ribbon Replacement Option exercise prices. The Ribbon Replacement Options are vesting under the same schedules as the respective Edgewater Options.

The fair values of the Edgewater Options assumed were estimated using a Black-Scholes option pricing model. The Company recorded $0.7 million as additional purchase consideration for the fair value of the Edgewater Options. The fair value of the Ribbon Replacement Options attributable to future service totaled $1.0 million, which is being recognized over a weighted average period of approximately two years.

2012 Stock Incentive Plan

In connection with the acquisition of Performance Technologies, Inc. ("PT"), the Company assumed PT's 2012 Amended Performance Technologies, Incorporated Omnibus Incentive Plan, and subsequently renamed it the 2012 Stock Incentive Plan (the "2012 Plan"). In December 2014, all of the unissued shares under the 2012 Plan were transferred to the 2007 Plan. Any outstanding awards under the 2012 Plan that in the future expire, terminate, are canceled, surrendered or forfeited, or are repurchased by the Company will be returned to the 2019 Plan. Accordingly, at December 31, 2019 there were no shares available for future issuance under the 2012 Plan.

2008 Stock Incentive Plan

In connection with the acquisition of Network Equipment Technologies, Inc. ("NET"), the Company assumed NET's 2008 Equity Incentive Plan and subsequently renamed it the 2008 Stock Incentive Plan (the "2008 Plan"). In December 2014, all of the unissued shares under the 2008 Plan were transferred to the 2007 Plan. Any outstanding awards under the 2008 Plan that in the future expire, terminate, are canceled, surrendered or forfeited, or are repurchased by the Company will be returned to the 2019 Plan. Accordingly, at December 31, 2019 there were no shares available for future issuance under the 2008 Plan.

Treatment of Equity Awards in Connection with the Merger

In connection with the Merger, the Company accelerated the vesting of all then-outstanding stock options and certain then-outstanding full value awards on the Merger Date. In addition, the vesting schedules of certain remaining unvested full value awards were adjusted. Such vesting and adjustments are described below:

Stock options - each stock option outstanding as of five business days prior to the Merger Date became vested in full as of that date (to the extent not previously vested), and the holders of such stock options were permitted to exercise their stock options from October 20, 2017 through October 24, 2017, after which date all remaining stock options, with certain exceptions, were canceled. The Company accelerated the vesting of 0.3 million stock options and subsequently canceled 4.5 million vested unexercised stock options in connection with this transaction. Any stock options granted under the 2008 Plan and 2012 Plan were not canceled, as these plans do not permit such cancellations. These stock options continue to be outstanding until they are either exercised or expire.

RSAs and RSUs - as prescribed by the Company's 2007 Plan, any unvested RSAs and RSUs that were scheduled to vest within one year from the Merger Date vested in full as of the Merger Date. The vesting schedules of the remaining unvested RSAs and RSUs were then accelerated by one year. Certain executives had specific terms and conditions related to their RSAs detailed in their employment agreements or amendments thereto (the "employment terms"). The accelerated vesting of and future vesting schedule adjustments to the RSAs held by these individuals were completed in accordance with their individual employment terms. In accordance with the terms of their RSA grants, unvested RSAs held by the then-current members of the Board were accelerated on a pro rata basis based on the amount of time the unvested RSAs were outstanding compared to the originally scheduled vesting date. Unvested PSUs granted to the Company's then-current President and Chief Executive Officer, who separated from the Company effective December 13, 2017 (the "Former CEO"), were converted to RSAs in accordance with his employment terms; certain of those converted grants were accelerated, and the remaining RSAs continued to vest according to their terms, but with the elimination of any required satisfaction of the performance metrics associated with the awards when they were originally granted as PSUs. In total, the Company accelerated the vesting of and released 1.1 million RSAs and approximately 36,000 RSUs in connection with the Merger.

PSUs - any unvested PSUs were accelerated in accordance with the employment agreement of each individual PSU holder. The remaining unvested units would continue to vest according to their terms, with the exception of the PSU grants held by the Former CEO, as discussed above. The Company accelerated the vesting of and released approximately 98,000 PSUs in connection with the Merger.

Executive Equity Arrangements

Stock-for-Cash Bonus Election

In connection with the Company’s annual incentive program, certain executives of the Company were given the choice to receive a portion, ranging from 10% to 50% (the "Elected Percentage") of their fiscal year 2018 bonuses (the “2018 Bonus”), if any were earned, in the form of shares of the Company’s common stock (the “2018 Bonus Shares” and such program, the “Stock Bonus Election Program”). Each executive could also elect not to participate in this program and to earn his or her 2018 Bonus, if any, in the form of cash. Any executive who elected to receive a portion of his or her 2018 Bonus in stock would also receive an uplift of 20% of the value of the 2018 Bonus Shares in additional shares of the Company’s common stock (the “Uplift Shares”) with the exception of the Company’s Chief Executive Officer and his senior leadership team. Under the Stock Bonus Election Program, the amount of the 2018 Bonus, if any, for each executive was determined by the Compensation Committee of the Board (the "Compensation Committee").

The number of shares earned by each of the 23 participants in the Stock Bonus Election Program was calculated by multiplying each participant's 2018 Bonus by the applicable Elected Percentage (plus the amount attributable to Uplift Shares, if applicable) and dividing the resulting amount by $4.97, the closing price of the Company’s common stock on March 8, 2019, the date of the company-wide cash bonus payments. The Company granted 198,949 shares in the aggregate in connection with the 2018 Bonus Shares on March 15, 2019, and such shares were fully vested on the date of grant. However, notwithstanding that each such share of common stock was fully vested, each participant in the Stock Bonus Election Program was contractually restricted from trading the 2018 Bonus Shares for five months after the date of grant. Both the grant and vesting of the 2018 Bonus Shares are included in the RSU table below.

The Company determined that the grant date criteria for the 2018 Bonus Shares and Uplift Shares had not been met as of December 31, 2018, as the number of shares to be granted to each executive had not been determined. The Company recorded stock-based compensation expense totaling $1.1 million in connection with the Stock Bonus Program in 2018 and recorded a liability in connection with the future issuance of the 2018 Bonus Shares and Uplift Shares. This liability was reclassified to equity at the time the shares were granted.

Performance-Based Stock Grants

In addition to granting RSAs and RSUs to its executives and certain of its employees, the Company also grants PSUs to certain of its executives.

2019 PSU Grants. In March and April 2019, the Company granted certain of its executives an aggregate of 872,073 PSUs, of which 523,244 PSUs had both performance and service conditions (the "Performance PSUs") and 348,829 PSUs had both market and service conditions (the "Market PSUs").

Each executive's Performance PSU grant is comprised of three consecutive fiscal year performance periods from 2019 through 2021 (each, a "Fiscal Year Performance Period"), with one-third of the Performance PSUs attributable to each Fiscal Year Performance Period. The number of shares that will vest for each Fiscal Year Performance Period will be based on the achievement of certain metrics related to the Company's financial performance for the applicable year on a standalone basis (each, a "Fiscal Year Performance Condition"). In the third quarter of 2019, the Company adjusted the 2019 Performance PSU goals to reflect the changes to the Company's calculation of certain metrics. There was no incremental expense in connection with this modification. The Company's achievement of the 2019 Fiscal Year Performance Conditions (and the number of shares of Company common stock to vest as a result thereof) will be measured on a linear sliding scale in relation to specific threshold, target and stretch performance conditions. The Company is recording stock-based compensation expense for the Performance PSUs based on its assessment of the probability that each performance condition will be achieved and the level, if any, of such achievement. As of December 31, 2019, the Company determined that the grant date criteria for the 2020 and 2021 Fiscal Year Performance Periods had not been met, as the 2020 and 2021 Fiscal Year Performance Conditions had not been established by the Company. Accordingly, the stock-based compensation expense recorded in the year ended December 31, 2019 in connection with the Performance PSUs is related only to those PSUs with 2019 Fiscal Year Performance Conditions. The Compensation Committee will determine the number of shares earned, if any, after the Company's financial results for each Fiscal Year Performance Period are finalized. Upon the determination by the Compensation Committee of the number of shares that will be received upon vesting of the Performance PSUs, such number of shares will become fixed and the unamortized expense will be recorded through the remainder of the service period that ends on March 15, 2022, at which time the total Performance PSUs earned, if any, will vest, pending each executive's continued employment with the Company through that date. The number of shares of common stock to be achieved upon vesting of the Performance PSUs will in no event exceed 200% of the Performance PSUs. Shares subject to the Performance PSUs that fail to be earned will be forfeited.

The Market PSUs have one three-year performance period, which will end on December 31, 2021 (the "Market Performance Period"). The number of shares subject to the Market PSUs that will vest, if any, on March 15, 2022, will be dependent upon the Company's total shareholder return ("TSR") compared with the TSR of the companies included in the Nasdaq Telecommunications Index for the same Market Performance Period, measured by the Compensation Committee after the Market Performance Period ends. The shares determined to be earned will vest on March 15, 2022, pending each executive's continued employment with the Company through that date. The number of shares of common stock to be achieved upon vesting of the Market PSUs will in no event exceed 200% of the Market PSUs. Shares subject to the Market PSUs that fail to be earned will be forfeited.

2018 PSU Grant. In May 2018, the Company granted its then-current President and Chief Executive Officer, Mr. Hobbs, 195,000 PSUs with both performance and service conditions (the “2018 PSUs”). Of the 195,000 2018 PSUs, one-half each would vest based on the achievement of two separate metrics related to the Company’s 2018 financial performance (the “2018 Performance Conditions”). The Company’s achievement of the 2018 Performance Conditions (and the shares of Company common stock to vest as a result thereof) were measured on a linear sliding scale in relation to specific threshold, target and stretch performance conditions. The number of shares of common stock to be received upon vesting of the 2018 PSUs would in no event exceed 150% of the 2018 PSUs. In the year ended December 31, 2018, the Company recorded stock-based compensation expense for the 2018 PSUs based on its assessment of the probability that each performance condition would be achieved and the level, if any, of such achievement. The Company recorded $0.3 million of stock-based compensation expense in the year ended December 31, 2018 in connection with the 2018 PSUs. In February 2019, the Compensation Committee determined that the performance metrics for one-half of the 2018 PSUs had been achieved at the 106.49% achievement level and one-half of the 2018 PSUs had been achieved at the 150% level, for a total of 250,075 shares earned. However, in April 2019, the Compensation Committee subsequently determined that the performance metrics for the entire 2018 PSUs had been achieved at the 150% level, for a total of 292,500 shares eligible to be issued (the "2018 Shares Earned"), pending Mr. Hobbs' continued employment with the Company through December 31, 2020. In connection with Mr. Hobbs' separation from the Company effective December 31, 2019, the vesting schedule for the 2018 Shares Earned was accelerated and the shares were released on January 30, 2020 (see "Accelerated Vesting of Unvested Stock Units Held by Mr. Hobbs" below).

2017 PSU Grants. On March 31, 2017, the Company granted an aggregate of 165,000 PSUs with both market and service conditions to five of its executives (the "2017 PSUs"). The terms of each PSU grant were such that up to one-third of the shares subject to the respective PSU grant would vest, if at all, on each of the respective first, second and third anniversaries of the date of grant, depending on the Company's TSR compared with the TSR of the companies included in the Nasdaq Telecommunications Index for the same fiscal year, measured by the Compensation Committee after each of the fiscal years as defined by each grant (each, a "Performance Period"). The shares determined to be earned would vest on the anniversary of the grant date following each Performance Period. Shares subject to the PSUs that failed to be earned would be forfeited. In March 2018, the Compensation Committee determined that the performance metrics for the 2017 PSUs for the 2017 Performance Period had been achieved at the 130% level and accordingly, 33,584 shares in the aggregate were released to the three remaining executives holding such outstanding grants, comprised of 25,834 shares, representing the 100% achievement of target, granted on March 31, 2017, and 7,750 shares, representing the 30% achievement over target, granted on March 31, 2018. In February 2019, the Compensation Committee determined that the performance metrics for the 2017 PSUs for the 2018 Performance Period had been achieved at the 61.4% level and accordingly, 9,464 shares were released to the three remaining executives holding such outstanding grants on March 31, 2019. The shares that failed to be earned for the 2018 Performance Period, aggregating 5,950 shares, were forfeited. Accordingly, at December 31, 2019, there were no remaining unvested 2017 PSUs outstanding. The release and forfeiture of the shares related to the 2018 Performance Period are included in the PSU table below.

PSUs that include a market condition require the use of a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the volatility of each entity and the pair-wise covariance between each entity. These results are then used to calculate the grant date fair values of the respective PSUs. The Company is required to record expense for the PSUs with market conditions through their respective final vesting dates regardless of the number of shares that are ultimately earned.

Accelerated Vesting of Unvested Stock Units Held by Mr. Hobbs. On November 13, 2019, the Company announced that, effective that date, Mr. Hobbs was no longer serving as the President and Chief Executive Officer of Ribbon, and, also on and effective November 13, 2019, the Board appointed Steven Bruny, the Company's Executive Vice President, Global Sales & Services, and Kevin Riley, the Company's Executive Vice President, Advanced R&D and Chief Technology Officer, as Interim Co-Presidents and Chief Executive Officers, to assume the duties of the Company's President and Chief Executive Officer while the Board searched for a permanent successor to Mr. Hobbs. Mr. Hobbs resigned his position as a member of the Board effective December 27, 2019 and separated from the Company effective December 31, 2019. In connection with his separation from the Company and in accordance with the terms of his employment agreement with the Company, the vesting of certain of Mr. Hobbs' RSUs and PSUs were accelerated, some of which were released on January 30, 2020, with certain other PSUs, if any are earned, scheduled for release no later than December 15, 2020.

Stock Options

Stock options granted under the 2019 Plan expire ten years from the date of grant. Outstanding stock options granted under the Edgewater Plan expire ten years from the date of grant. Outstanding stock options granted under the 2008 Plan expire either seven or ten years from the date of grant. Outstanding stock options granted under the 2012 Plan expire ten years from the date of grant. There were no stock options outstanding under either the 2019 Plan or the 2007 Plan at December 31, 2019. The grant date fair value of stock options, adjusted for estimated forfeitures, is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. Forfeitures are estimated based on historical experience.

The activity related to the Company's outstanding stock options during the year ended December 31, 2019 was as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2019	582,061		$	9.01
Granted	—		$	—
Exercised	(127,334	)	$	1.85
Forfeited	(38,666	)	$	2.55
Expired	(118,937	)	$	12.43
Outstanding at December 31, 2019	297,124		$	11.55	4.95	$	122
Vested or expected to vest at December 31, 2019	293,782		$	11.65	4.92	$	119
Exercisable at December 31, 2019	260,152		$	12.89	4.59	$	85

The Company did not grant stock options during the years ended December 31, 2019 and 2018. The grant date fair values of stock options granted in the year ended December 31, 2017 were estimated using the Black-Scholes valuation model with the following assumptions:



Risk-free interest rate		1.22% - 1.95%
Expected dividends		—
Weighted average volatility		51.1%
Expected life (years)		5.0

The risk-free interest rate used is the average U.S. Treasury Constant Maturities Rate for the expected life of the award. The expected dividend yield of zero is based on the fact that the Company has never paid dividends and has no present intention to pay cash dividends. The expected life for stock options is based on a combination of the Company's historical option patterns and expectations of future employee actions.

The weighted average grant-date fair value of options granted for the year ended December 31, 2017 was $3.05.

The total intrinsic values of options exercised were $0.5 million for the year ended December 31, 2019, approximately $39,000 for the year ended December 31, 2018 and $0.2 million for the year ended December 31, 2017.

The Company received cash from option exercises of $0.2 million in the year ended December 31, 2019, $0.1 million in the year ended December 31, 2018 and $0.6 million in the year ended December 31, 2017.

Restricted Stock Grants - Restricted Stock Awards and Restricted Stock Units

The Company's outstanding restricted stock grants consist of both RSAs and RSUs. Holders of unvested RSAs have voting rights and rights to receive dividends, if declared; however, these rights are forfeited if the underlying unvested RSA shares are forfeited. Holders of unvested RSUs do not have such voting and dividend rights. The grant date fair value of restricted stock grants, adjusted for estimated forfeitures, is recognized as expense on a straight-line basis over the requisite service period. The fair value of restricted stock grants is determined based on the market value of the Company's shares on the date of grant.

The activity related to the Company's RSAs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	1,508,011		$	6.90
Granted	—		$	—
Vested	(967,291	)	$	6.90
Forfeited	(52,744	)	$	7.04
Unvested balance at December 31, 2019	487,976		$	6.87

The activity related to the Company's RSUs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	636,300		$	6.52
Granted	2,828,832		$	4.98
Vested	(537,416	)	$	6.04
Forfeited	(137,656	)	$	5.35
Unvested balance at December 31, 2019	2,790,060		$	5.11

The total grant date fair value of vested restricted stock grant shares was $9.9 million in the year ended December 31, 2019, $9.7 million in the year ended December 31, 2018 and $19.1 million in the year ended December 31, 2017.

Performance-Based Stock Units

Holders of unvested PSUs do not have voting and dividend rights. The Company recognizes stock-based compensation expense for PSUs without market conditions on a straight-line basis, with the amount recorded based upon the expected level of achievement as of each period-end, recording cumulative adjustments in the period when the expected level of achievement changes. The Company recognizes the grant date fair value of PSUs on a graded attribution basis through the vest date of the respective awards so long as it remains probable that the related service conditions will be satisfied.

The activity related to the Company's PSUs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	210,416		$	5.77
Granted	872,073		$	6.03
Vested	(9,466	)	$	8.55
Forfeited	(5,950	)	$	8.55
Unvested balance at December 31, 2019	1,067,073		$	5.94

The total grant date fair value of vested performance-based stock grant shares was $0.1 million in the year ended December 31, 2019, $0.6 million in the year ended December 31, 2018 and $1.4 million in the year ended December 31, 2017.

ESPP

The ESPP is designed to provide eligible employees of the Company and its participating subsidiaries an opportunity to purchase common stock of the Company through accumulated payroll deductions.

The ESPP provides for six-month consecutive offering periods, with the purchase price of the stock equal to 85% of the lesser of the market price on the first or last day of the offering period. The maximum number of shares of common stock an employee may purchase during each offering period is 500, subject to certain adjustments pursuant to the ESPP.

In May 2017, the Compensation Committee determined to suspend all offering periods under the ESPP, effective September 1, 2017, until such time after the Merger Date as the Compensation Committee determined was best in its sole discretion. The Company's Board voted to re-implement the ESPP effective December 1, 2018 for employees in certain geographic regions, with the first purchase date of the re-implemented ESPP completed on May 31, 2019.

At December 31, 2019, 5.0 million shares, the maximum number of shares that may be issued under the ESPP, were authorized and 1.1 million shares were available under the ESPP for future issuance. The ESPP will expire on May 20, 2020.

Stock-Based Compensation

The consolidated statements of operations included stock-based compensation for the years ended December 31, 2019, 2018 and 2017 as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Product cost of revenue	$	76	$	114	$	514
Service cost of revenue	478		345		1,448
Research and development	1,898		1,797		7,337
Sales and marketing	3,028		2,935		4,885
General and administrative	7,121		5,881		11,473
	$	12,601	$	11,072	$	25,657

There was no income tax benefit for employee stock-based compensation expense for the years ended December 31, 2019, 2018 and 2017 due to the valuation allowance recorded.

Stock-based compensation expense recorded for the year ended December 31, 2019 included $1.6 million of incremental expense related to the accelerated vesting of RSUs and PSUs held by the former President and Chief Executive Officer in connection with his separation from the Company effective December 31, 2019. Stock-based compensation expense recorded for the year ended December 31, 2017 included $8.6 million of incremental expense related to the acceleration of stock options and full value awards and subsequent adjustments to the vesting schedules of the remaining unvested full value awards in connection with the Merger. In addition, the Company recorded $1.6 million of incremental expense related to the accelerated vesting of RSAs held by a former President and Chief Executive Officer in connection with his separation from the Company effective December 13, 2017.

At December 31, 2019, there was $11.3 million, net of expected forfeitures, of unrecognized stock-based compensation expense related to unvested stock options, RSAs, RSUs and PSUs. This expense is expected to be recognized over a weighted average period of approximately two years.

Common Stock Reserved

Common stock reserved for future issuance at December 31, 2019 consists of the following:



2019 Plan	7,051,559
ESPP	1,148,867
	8,200,426

The Company's policy is to issue authorized but unissued shares upon the exercise of stock options, to grant restricted common stock, to settle restricted stock units and performance-based stock units, and to authorize the purchase of shares of the Company's common stock under the ESPP.

LEASES

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

LEASES

The Company has operating and finance leases for corporate offices, research and development facilities, and certain equipment. Operating leases are reported separately in the Company's consolidated balance sheet at December 31, 2019. Assets acquired under finance leases are included in Property and equipment, net, in the consolidated balance sheets at December 31, 2019 and 2018.

The Company determines if an arrangement is a lease at inception. A contract is determined to contain a lease component if the arrangement provides the Company with a right to control the use of an identified asset. Lease agreements may include lease and non-lease components. In such instances for all classes of underlying assets, the Company does not separate lease and non-lease components but rather, accounts for the entire arrangement under leasing guidance. Leases with an initial term of 12 months or less are not recorded on the balance sheet and lease expense for these leases is recognized on a straight-line basis over the lease term.

Right-of-use assets and lease liabilities are initially measured based on the present value of the future minimum fixed lease payments (i.e., fixed payments in the lease contract) over the lease term at the commencement date. As the Company's existing leases do not have a readily determinable implicit rate, the Company uses its incremental borrowing rate based on the information available at the commencement date in determining the present value of future minimum fixed lease payments. The Company calculates its incremental borrowing rate to reflect the interest rate that it would have to pay to borrow on a collateralized basis an amount equal to the lease payments in a similar economic environment over a similar term and considers its historical borrowing activities and market data from entities with comparable credit ratings in this determination. The measurement of the right-of-use asset also includes any lease payments made prior to the commencement date (excluding any lease incentives) and initial direct costs incurred. The Company assessed its right-of-use assets for impairment as of December 31, 2019 and determined no impairment has occurred.

Lease terms may include options to extend or terminate the lease and the Company incorporates such options in the lease term when it has the unilateral right to make such an election and it is reasonably certain that the Company will exercise that option. In making this determination, the Company considers its prior renewal and termination history and planned usage of the assets under lease, incorporating expected market conditions.

For operating leases, lease expense for minimum fixed lease payments is recognized on a straight-line basis over the lease term. The expense for finance leases includes both interest and amortization expense components, with the interest component calculated based on the effective interest method and the amortization component calculated based on straight-line amortization of the right-of-use asset over the lease term. Lease contracts may contain variable lease costs, such as common area maintenance, utilities and tax reimbursements that vary over the term of the contract. Variable lease costs are not included in minimum fixed lease payments and as a result, are excluded from the measurement of the right-of-use assets and lease liabilities. The Company expenses all variable lease costs as incurred.

In connection with the 2019 Restructuring Initiative, certain lease assets related to facilities will be partially or fully vacated as the Company consolidates its facilities. The Company has no plans to enter into sublease agreements for certain facilities. The Company ceased use of these facilities in the third quarter of 2019. Accordingly, the Company accelerated the amortization of the associated lease assets through the planned cease-use date of each facility, resulting in additional amortization expense of $3.7 million in the year ended December 31, 2019. The Company also recorded a liability of $0.9 million in 2019 for all future anticipated variable lease costs related to these facilities. This incremental accelerated amortization and estimated future variable lease costs are included in Restructuring and related expense in the Company's consolidated statements of operations for the year ended December 31, 2019. The Company may incur additional future expense if it is unable to sublease other locations included in the Facilities Initiative.

The Company leases its corporate offices and other facilities under operating leases, which expire at various times through 2029. The Company's corporate headquarters is located in a leased facility in Westford, Massachusetts under a lease that expires in August 2028. The Company's finance leases primarily consist of equipment.

At December 31, 2019, the Company had 107,800 square feet of building space in North Dallas, Texas under construction as part of its 2019 Restructuring and Facilities Consolidation Initiative. The Company's leased Plano, Texas facility will be vacated upon completion of the construction of the North Dallas, Texas site. At that time, employees will relocate to the new site as part of the 2019 Restructuring Initiative. The construction of the new North Dallas, Texas site is expected to be completed in 2020.

The Company's right-of-use lease assets and lease liabilities at December 31, 2019 and December 31, 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Assets
Operating lease assets	$	36,654	$	—
Finance lease assets*	2,420		2,104
Total leased assets	$	39,074	$	2,104

Liabilities
Current
Operating	$	7,719	$	—
Finance	1,005		1,039
Noncurrent
Operating	37,202		—
Finance	2,144		1,324
Total lease liabilities	$	48,070	$	2,363

* Finance lease assets were recorded net of accumulated depreciation of $2.0 million and $0.9 million at December 31, 2019 and December 31, 2018, respectively, and were reported as capital lease assets prior to the Company's adoption of ASC 842.

The components of lease expense for the year ended December 31, 2019 were as follows (in thousands):




Operating lease cost*	$	13,865
Finance lease cost
Amortization of leased assets	1,106
Interest on lease liabilities	265
Short-term lease cost	19,460
Variable lease costs (costs excluded from minimum fixed lease payments)**	3,264
Sublease income	(374		)
Net lease cost	$	37,586

* Operating lease cost for the year ended December 31, 2019 includes $3.7 million of accelerated amortization for certain assets partially or fully vacated in 2019 with no intent or ability to sublease.

** Variable lease costs for the year ended December 31, 2019 include a $0.9 million accrual for all future estimated variable expenses related to certain assets partially or fully vacated in 2019 with no intent or ability to sublease.

The Company elected to use the alternative transition method, which allows entities to initially apply ASC 842 at the adoption date with no subsequent adjustments to prior period lease costs for comparability. As a result, operating leases in periods prior to the Company's adoption of ASC 842 were not recorded on the consolidated balance sheet. Prior to the adoption of ASC 842, rent expense (including any escalation clauses, free rent and other lease concessions) on operating leases was recognized on a straight-line basis over the minimum lease term, and this remains consistent with the Company's application of ASC 842. Rent expense was $11.9 million and $5.9 million for the years ended December 31, 2018 and 2017, respectively. Interest expense for finance leases was approximately $52,000 and $14,000 for the years ended December 31, 2018 and 2017, respectively. Amortization expense for finance leases was $0.6 million and $0.1 million for the years ended December 31, 2018 and 2017, respectively.

Other information related to the Company's leases as of and for the year ended December 31, 2019 was as follows (in thousands, except lease terms and percentages):



Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	10,559
Operating cash flows from finance leases	$	265
Financing cash flows from finance leases	$	913
Weighted average remaining lease term (years):
Operating leases	6.73
Finance leases	2.35
Weighted average discount rate:
Operating leases	6.50		%
Finance leases	7.54		%

Future minimum fixed lease payments under noncancelable leases at December 31, 2019 were as follows (in thousands):



	Operating			Finance
	leases			leases
2020	$	10,290		$	1,644
2021	9,468			1,159
2022	7,665			581
2023	7,067			—
2024	5,303			—
2025 and beyond	15,738			—
Total lease payments	55,531			3,384
Less: interest	(10,610		)	(235		)
Present value of lease liabilities	$	44,921		$	3,149

Future minimum fixed lease payments under noncancelable leases at December 31, 2018 were as follows (in thousands):



	Operating		Finance
	leases*		leases
2019	$	10,705	$	1,386
2020	8,384		1,010
2021	7,455		288
2022	5,691		—
2023	5,430		—
2024 and beyond	19,818		—
Total lease payments	$	57,483	2,684
Less: interest			(321		)
Present value of lease liabilities**			$	2,363

* The amounts in this column include restructuring payments aggregating approximately $1 million, of which approximately 50% was due in less than one year and the remainder was due in one to three years. These amounts exclude current estimated sublease income aggregating approximately $125,000 over the remaining lease terms for restructured facilities.

** Prior to the Company's adoption of ASC 842 on January 1, 2019, operating leases were not recorded on the consolidated balance sheet and no interest component was calculated.

LEASES

The Company's right-of-use lease assets and lease liabilities at December 31, 2019 and December 31, 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Assets
Operating lease assets	$	36,654	$	—
Finance lease assets*	2,420		2,104
Total leased assets	$	39,074	$	2,104

Liabilities
Current
Operating	$	7,719	$	—
Finance	1,005		1,039
Noncurrent
Operating	37,202		—
Finance	2,144		1,324
Total lease liabilities	$	48,070	$	2,363

The components of lease expense for the year ended December 31, 2019 were as follows (in thousands):




Operating lease cost*	$	13,865
Finance lease cost
Amortization of leased assets	1,106
Interest on lease liabilities	265
Short-term lease cost	19,460
Variable lease costs (costs excluded from minimum fixed lease payments)**	3,264
Sublease income	(374		)
Net lease cost	$	37,586

Other information related to the Company's leases as of and for the year ended December 31, 2019 was as follows (in thousands, except lease terms and percentages):



Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	10,559
Operating cash flows from finance leases	$	265
Financing cash flows from finance leases	$	913
Weighted average remaining lease term (years):
Operating leases	6.73
Finance leases	2.35
Weighted average discount rate:
Operating leases	6.50		%
Finance leases	7.54		%

Future minimum fixed lease payments under noncancelable leases at December 31, 2019 were as follows (in thousands):



	Operating			Finance
	leases			leases
2020	$	10,290		$	1,644
2021	9,468			1,159
2022	7,665			581
2023	7,067			—
2024	5,303			—
2025 and beyond	15,738			—
Total lease payments	55,531			3,384
Less: interest	(10,610		)	(235		)
Present value of lease liabilities	$	44,921		$	3,149

Future minimum fixed lease payments under noncancelable leases at December 31, 2018 were as follows (in thousands):



	Operating		Finance
	leases*		leases
2019	$	10,705	$	1,386
2020	8,384		1,010
2021	7,455		288
2022	5,691		—
2023	5,430		—
2024 and beyond	19,818		—
Total lease payments	$	57,483	2,684
Less: interest			(321		)
Present value of lease liabilities**			$	2,363

** Prior to the Company's adoption of ASC 842 on January 1, 2019, operating leases were not recorded on the consolidated balance sheet and no interest component was calculated.

EMPLOYEE DEFINED CONTRIBUTION PLANS

12 Months Ended

Dec. 31, 2019

Retirement Benefits [Abstract]

EMPLOYEE DEFINED CONTRIBUTION PLANS

The Company offers 401(k) savings plans to eligible employees. The Company assumed GENBAND's 401(k) savings plan in connection with the Merger. Effective January 1, 2019, the previously separate former Sonus and former GENBAND 401(k) savings plans were combined into one plan.

Effective January 1, 2018, the Company began to match 50% of each employee's contributions to the 401(k) program up to 4% of the employee's eligible earnings, for a maximum match of 2% of eligible earnings.

The Company recorded expense related to its employee defined contribution plans aggregating $4.0 million in the year ended December 31, 2019, $3.2 million in the year ended December 31, 2018 and $1.4 million in the year ended December 31, 2017.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

In connection with the Merger, the Company assumed GENBAND's defined benefit retirement plans that cover certain employees at various international locations. The Company adopted GENBAND's policy to contribute amounts at least sufficient to satisfy the minimum amount required by applicable law and regulations or to directly pay benefits where appropriate. Benefits under the defined benefit plans are typically based either on years of service and the employee's compensation (generally during a fixed number of years immediately before retirement) or on annual credits. The range of assumptions that are used for the non-U.S. defined benefit plans reflect the different economic environments within the various countries.

During the year ended December 31, 2019, in conjunction with the 2019 Restructuring Initiative, there were reductions in force that significantly reduced benefits that can be earned under the plan in one of our international locations that resulted in an immaterial curtailment loss. Settlement accounting was triggered in the year ended December 31, 2019 related to a reduction in force in one of our locations in 2018, resulting in an immaterial settlement gain.

During the year ended December 31, 2018, in conjunction connection with the Merger Restructuring Initiative, there were reductions in force that significantly reduced benefits that can be earned under the defined benefit plans in several international locations that resulted in curtailment accounting. A curtailment gain of $0.5 million was recognized in 2018 and included as a component of Other (expense) income, net, in the Company's consolidated statement of operations. In the year ended December 31, 2018, settlement accounting was triggered in only one of these locations, resulting in an immaterial settlement charge.

A reconciliation of the changes in the benefit obligations and fair value of the assets of the defined benefit plans for the years ended December 31, 2019 and 2018, the funded status of the plans, and the amounts recognized in the consolidated balance sheets as of December 31, 2019 and 2018 were as follows (in thousands):



	Year ended December 31, 2019			Year ended December 31, 2018
Changes in projected benefit obligations:
Projected benefit obligation, beginning of year	$	10,848		$	11,484
Service cost	335			449
Interest cost	140			150
Participant contributions	24			5
Benefits and expenses paid	(44		)	(23		)
Net actuarial loss (gain) on obligation	1,059			(414		)
Curtailment	82			(553		)
Settlement	(660		)	(250		)
Projected benefit obligation, end of year	$	11,784		$	10,848

Changes in plan assets:
Fair value of plan assets, beginning of year	$	3,842		$	3,893
Actual return on plan assets	(1,471		)	(53		)
Employer contributions	139			292
Participant contributions	24			5
Administrative expenses	(21		)	(22		)
Benefits paid	(683		)	(273		)
Fair value of plan assets, end of year	$	1,830		$	3,842

Funded status at end of year	$	(9,954	)	$	(7,006	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	2,743		$	222

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(74	)	$	(75	)
Other long-term liabilities (non-current pension liability)	(9,880		)	(6,931		)
Net amount recognized	$	(9,954	)	$	(7,006	)

The increase in the underfunded status of the Company's defined benefit plans at December 31, 2019 compared to December 31, 2018 was the result of asset losses and a general decrease in discount rates which resulted in an increase in the projected benefit obligation.

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2019 and 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Aggregate projected benefit obligation	$	11,784	$	10,848
Aggregate accumulated benefit obligation	$	7,759	$	7,152
Aggregate fair value of plan assets	$	1,830	$	3,842

Net periodic benefit costs for the years ended December 31, 2019 and 2018 and the period from the Merger Date to December 31, 2017 were as follows (in thousands):



	Year ended December 31, 2019			Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	335		$	449		$	68
Interest cost	140			150			25
Expected return on plan assets	(14		)	(45		)	(8		)
Plan asset expenses	21			22			4
Curtailment charge (credit)	13			(510		)	—
Settlement charge	115			3			—
Net periodic benefit costs	$	610		$	69		$	89

The Company made benefit payments of $683,000 and $273,000 in the years ended December 31, 2019 and 2018, respectively. These benefit payments included $660,000 and $250,000 of one-time lump sum payments to participants in 2019 and 2018, respectively. The Company made benefit payments of $3,000 in the period from the Merger Date to December 31, 2017. Expected benefit payments for the next ten years are as follows (in thousands):



Years ending December 31,
2020	$	74
2021	94
2022	45
2023	223
2024	57
2025 to 2029	1,661
	$	2,154

The changes in plan assets and benefit obligations recognized in other comprehensive income (loss) before tax for the years ended December 31, 2019 and 2018 and the period from the Merger Date to December 31, 2017 were as follows (in thousands):



	Year ended December 31, 2019		Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Net loss (gain)	$	2,526	$	(356	)	$	578

The Company defers all actuarial gains and losses resulting from variances between actual results and economic estimates or actuarial assumptions. The unrecognized actuarial gains and losses are recorded as unrealized pension actuarial gains (losses) in the Company's consolidated balance sheets as a component of Accumulated other comprehensive income. These unrecognized gains and losses are amortized as a component of net periodic benefit cost when the net gains and losses exceed 10% of the greater of the market value of plan assets or the projected benefit obligation at the beginning of the year. Amortization of the amount included in Accumulated other comprehensive income into net periodic benefit cost is expected to total approximately $176,000 for the year ended December 31, 2020.

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2019 and 2018 were as follows:



	December 31, 2019		December 31, 2018
Discount rate	0.68	%	1.30	%
Rate of compensation increase	2.88	%	2.83	%

The principal weighted average assumptions used to determine net period benefit cost for the years ended December 31, 2019 and 2018 and the period from the Merger Date to December 31, 2017 were as follows:



	Year ended December 31, 2019		Year ended December 31, 2018		October 27, 2017 to December 31, 2017
Discount rate	1.30	%	1.50	%	1.49	%
Expected long-term return on plan assets	1.12	%	1.34	%	1.23	%
Rate of compensation increase	2.83	%	3.38	%	3.38	%

Assumed discount rates are used in the measurement of the projected and accumulated benefit obligations, as well as the service and interest cost components of net periodic pension cost. Estimated discount rates reflect the rates at which the pension benefits could be effectively settled. For each defined benefit plan, the Company chooses an estimated discount rate from a readily available market index rate, based upon high-quality fixed income investments, specific to the country or economic zone in which the benefits are paid and taking into account the duration of the plan and the number of participants.

The plans in the Netherlands and Switzerland are funded through insurance contracts, which provide guaranteed interest credit. The fair value of the contract is derived from the insurance company's assessment of the minimum value of the benefits provided by the insurance contract. The methodology used to value the plan assets assumes that the value of the plan assets equals the guaranteed insured benefits. For consistency, the same discount rate used in the valuation of the benefit obligations is used to place a value on the plan assets. The assets are assumed to grow each year in line with the discount rate, and therefore, the expected return on the assets is set equal to the discount rate. The fair value of the combined plan assets was $1.8 million at December 31, 2019 and $3.8 million at December 31, 2018. The Company classifies the fair value of these plan assets as Level 2 in the fair value hierarchy as discussed in Note 5.

During the years ended December 31, 2019 and 2018, employees in the Netherlands and Switzerland made contributions to the respective pension plans aggregating $24,000 and $5,000, respectively. During the period from the Merger Date to December 31, 2017, employees in the Netherlands and Switzerland made contributions to the respective plans aggregating $5,000. Employee contributions to these plans are based on a fixed 5% of the relevant pensionable earnings. The Company funds these plans by contributing at least the minimum amount required by applicable regulations and as recommended by an independent actuary. During the years ended December 31, 2019 and 2018, the Company contributed $139,000 and $292,000, respectively, to its pension plans. During the period from the Merger Date to December 31, 2017, the Company contributed $22,000 to its pension plans. The Company expects to contribute $0.2 million to its pension plans in 2020.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

12 Months Ended

Dec. 31, 2019

Retirement Benefits [Abstract]

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

EMPLOYEE DEFINED CONTRIBUTION PLANS

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS



	Year ended December 31, 2019			Year ended December 31, 2018
Changes in projected benefit obligations:
Projected benefit obligation, beginning of year	$	10,848		$	11,484
Service cost	335			449
Interest cost	140			150
Participant contributions	24			5
Benefits and expenses paid	(44		)	(23		)
Net actuarial loss (gain) on obligation	1,059			(414		)
Curtailment	82			(553		)
Settlement	(660		)	(250		)
Projected benefit obligation, end of year	$	11,784		$	10,848

Changes in plan assets:
Fair value of plan assets, beginning of year	$	3,842		$	3,893
Actual return on plan assets	(1,471		)	(53		)
Employer contributions	139			292
Participant contributions	24			5
Administrative expenses	(21		)	(22		)
Benefits paid	(683		)	(273		)
Fair value of plan assets, end of year	$	1,830		$	3,842

Funded status at end of year	$	(9,954	)	$	(7,006	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	2,743		$	222

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(74	)	$	(75	)
Other long-term liabilities (non-current pension liability)	(9,880		)	(6,931		)
Net amount recognized	$	(9,954	)	$	(7,006	)

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2019 and 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Aggregate projected benefit obligation	$	11,784	$	10,848
Aggregate accumulated benefit obligation	$	7,759	$	7,152
Aggregate fair value of plan assets	$	1,830	$	3,842

Net periodic benefit costs for the years ended December 31, 2019 and 2018 and the period from the Merger Date to December 31, 2017 were as follows (in thousands):



	Year ended December 31, 2019			Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	335		$	449		$	68
Interest cost	140			150			25
Expected return on plan assets	(14		)	(45		)	(8		)
Plan asset expenses	21			22			4
Curtailment charge (credit)	13			(510		)	—
Settlement charge	115			3			—
Net periodic benefit costs	$	610		$	69		$	89



Years ending December 31,
2020	$	74
2021	94
2022	45
2023	223
2024	57
2025 to 2029	1,661
	$	2,154



	Year ended December 31, 2019		Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Net loss (gain)	$	2,526	$	(356	)	$	578

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2019 and 2018 were as follows:



	December 31, 2019		December 31, 2018
Discount rate	0.68	%	1.30	%
Rate of compensation increase	2.88	%	2.83	%



	Year ended December 31, 2019		Year ended December 31, 2018		October 27, 2017 to December 31, 2017
Discount rate	1.30	%	1.50	%	1.49	%
Expected long-term return on plan assets	1.12	%	1.34	%	1.23	%
Rate of compensation increase	2.83	%	3.38	%	3.38	%

INCOME TAXES

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

INCOME TAXES

The components of loss from continuing operations before income taxes consisted of the following (in thousands):



	Year ended December 31,
	2019			2018			2017
Income (loss) before income taxes:
United States	$	(132,887	)	$	(52,569	)	$	(55,932	)
Foreign	9,994			(20,841		)	2,240
	$	(122,893	)	$	(73,410	)	$	(53,692	)

The provision (benefit) for income taxes from continuing operations consisted of the following (in thousands):



	Year ended December 31,
	2019			2018			2017
Provision (benefit) for income taxes:
Current:
Federal	$	11		$	561		$	(200	)
State	128			128			115
Foreign	1,744			2,198			1,960
Total current	1,883			2,887			1,875
Deferred:
Federal	9,790			(8,481		)	49,570
State	1,630			(1,414		)	(4,833		)
Foreign	383			(1,477		)	(816		)
Change in valuation allowance	(6,504		)	11,885			(64,236		)
Total deferred	5,299			513			(20,315		)
Total	$	7,182		$	3,400		$	(18,440	)

A reconciliation of the Company's effective tax rate for continuing operations to the statutory federal rate is as follows:



	Year ended December 31,
	2019		2018		2017
U.S. statutory income tax rate	21.0	%	21.0	%	35.0	%
State income taxes, net of federal benefit	(0.2	)	(0.1	)	1.2
Foreign income taxes	(1.0	)	(5.7	)	(0.5	)
Acquisition costs	(0.5	)	(0.3	)	(6.0	)
Foreign deemed dividends	(0.4	)	(3.4	)	(3.8	)
Stock-based compensation	(0.7	)	(0.3	)	26.8
Tax credits	2.8		0.6		(33.3	)
Uncertain tax positions	(0.2	)	1.3		(1.2	)
NOL and credit limitations	—		—		(18.9	)
Valuation allowance	(0.7	)	(16.1	)	29.0
Goodwill amortization	0.4		0.3		(1.7	)
Meals and entertainment	(0.3	)	(0.4	)	(0.5	)
Tax reform	(0.1	)	—		8.8
Goodwill impairment	(25.4	)	—		—
Other, net	(0.5	)	(1.5	)	(0.6	)
Effective income tax rate	(5.8	)%	(4.6	)%	34.3	%

The following is a summary of the significant components of deferred income tax assets and liabilities (in thousands):



	December 31,
	2019			2018
Assets:
Net operating loss carryforwards	$	61,057		$	76,278
Research and development tax credits	32,879			28,664
Deferred revenue	7,868			5,755
Accrued expenses	5,687			9,601
Inventory	4,618			4,906
Stock-based compensation	2,880			1,536
Fixed assets	5,461			7,716
Other temporary differences	2,138			1,943
	122,588			136,399
Valuation allowance	(94,980		)	(101,484		)
Total deferred tax assets	27,608			34,915
Liabilities:
Purchased intangible assets	(22,470		)	(26,014		)
Unremitted foreign income	(4,827		)	(4,487		)
Total deferred tax liabilities	(27,297		)	(30,501		)
Total net deferred tax assets	$	311		$	4,414

The deferred tax assets and liabilities based on tax jurisdictions are presented in the Company's consolidated balance sheets as follows:
Deferred income taxes - noncurrent assets	$	4,959		$	9,152
Deferred income taxes - noncurrent liabilities	(4,648		)	(4,738		)
	$	311		$	4,414

At December 31, 2019, the Company had cumulative federal and state net operating losses ("NOLs") of $224.8 million. The federal NOL carryforwards expire at various dates from 2020 through 2037. The state NOL carryforwards expire at various dates from 2020 through 2038.

The Company also has available federal, state and foreign income tax credit carryforwards of $32.9 million that expire at various dates from 2020 through 2038.

Under the provisions of the Internal Revenue Code, the net operating losses and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating losses and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, as well as a similar state provision. As a result of the Edgewater Acquisition in 2018, the Company acquired approximately $34 million of net operating loss carryforwards and approximately $6 million of tax credit carryforwards. Edgewater incurred an ownership change as a result of its acquisition by the Company; however, the Company does not expect that any of the net operating losses or tax credits related to Edgewater will expire unused.

In connection with the Company's adoption of ASC 606, the Company recorded an adjustment to decrease its deferred tax assets by $2.2 million in 2018. There was no impact to the Company's consolidated financial statements for this adjustment due to the Company's full valuation allowance against these assets.

On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Act. The Tax Act makes broad and complex changes to the U.S. tax code, including, but not limited to: reducing the U.S. federal corporate tax rate from 35% to 21%; requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries; generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; requiring a current inclusion in U.S. federal taxable income of certain earnings (the Global Intangible Low-taxed Income ("GILTI")) of controlled foreign corporations; eliminating the corporate alternative minimum tax ("AMT") and changing how existing AMT credits can be realized; creating the base erosion anti-abuse tax ("BEAT"); creating a new limitation on deductible interest expense; changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017; providing a tax deduction for foreign derived intangible income ("FDII"); and changing rules related to deductibility of compensation for certain officers. The consequences of the Tax Act were reflected in the Company's U.S. tax provision for the year ended December 31, 2018 and the Company has completed its accounting for the effects of the Tax Act within the measurement period. The Company did not have any FDII or GILTI adjustments, but recorded a BEAT tax expense of $0.4 million and recorded an adjustment to the provisional amounts recorded at December 31, 2017 related to the Tax Act that decreased the Company's deferred tax assets by $0.2 million. These adjustments were recorded in the year ended December 31, 2018. When the 2018 Federal tax return was filed, there was no BEAT tax expense. The related true-up was recorded as a provision to return adjustment in the 2019 tax provision.

During 2019 and 2018, the Company performed an analysis to determine if, based on all available evidence, it considered it more likely than not that some portion or all of the recorded deferred tax assets will not be realized in a future period. As a result of the Company's evaluation, the Company concluded that there was insufficient positive evidence to overcome the more objective negative evidence related to its cumulative losses and other factors. Accordingly, the Company has maintained a valuation allowance against its domestic deferred tax asset amounting to $71.4 million at December 31, 2019 and $82.4 million at December 31, 2018. A similar analysis and conclusion were made with regard to the valuation allowance on the deferred tax assets of the Company's Ireland subsidiary, acquired as part of the acquisition of GENBAND, resulting in a valuation allowance of $9.2 million at December 31, 2019 and $9.5 million at December 31, 2018. In analyzing the deferred tax assets related to the Company's Canada subsidiaries at such time, the Company concluded that it was more likely than not that the Canadian federal credits would not be realized in a future period. This resulted in a valuation allowance of $11.0 million. In addition, because of continued losses, a valuation allowance of $2.7 million was established for the Company's Brazil subsidiary. The deferred tax assets recognized with no valuation allowance at December 31, 2019 and 2018 relate to foreign subsidiaries where recoverability is concluded to be more likely than not based on the Company's cost plus compensation policy as well as a state credit in the U.S.

A reconciliation of the Company's unrecognized tax benefits is as follows (in thousands):



	2019			2018			2017
Unrecognized tax benefits at January 1	$	3,461		$	4,528		$	8,969
Increases related to current year tax positions	292			74			139
Increases related to prior period tax positions	—			122			430
Increases related to business acquisitions	—			—			2,012
Decreases related to prior period tax positions	(821		)	(1,263		)	(7,022		)
Settlements	—			—			—
Unrecognized tax benefits at December 31	$	2,932		$	3,461		$	4,528

The Company recorded liabilities for potential penalties and interest of $0.1 million for the year ended December 31, 2019, $0.1 million for the year ended December 31, 2018 and $0.2 million for the year ended December 31, 2017. The Company had cumulative deferred tax liabilities recorded related to interest and penalties of $0.7 million for the year ended December 31, 2019, $0.6 million for the year ended December 31, 2018 and $0.6 million for the year ended December 31, 2017. Some of the unrecognized tax benefit items are expected to reverse in 2020 due to statute of limitation lapses.

The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction as well as various state and foreign jurisdictions. Generally, the tax years 2016 through 2019 remain open to examination by the major taxing jurisdictions to which the Company is subject. The Company's federal NOLs generated prior to 2016 could be adjusted on examination even though the year in which the loss was generated is otherwise closed by the statute of limitations.

As of December 31, 2019, the Company had ongoing income tax audits in certain foreign countries. Management believes that an adequate provision has been recorded for any adjustments that may result from tax examinations.

The Anova Acquisition was accounted for as a business combination and the financial results of Anova have been included in the Company's consolidated financial statements for the period subsequent to the Anova Acquisition Date. The transaction is considered an asset acquisition for tax purposes. Accordingly, Ribbon recorded a stepped up basis in the assets.

The Edgewater Acquisition was accounted for as a non-taxable business combination. Edgewater had previously been a single corporate filer for U.S. tax purposes. Consequently, U.S. federal and state deferred taxes were recorded as part of the business combination based on the differences between the tax basis of the acquired assets and assumed liabilities and their reported amounts for financial reporting purposes. The Company concluded that there was insufficient positive evidence to overcome the more objective negative evidence related to cumulative losses and other factors. The Company recorded identifiable intangible assets as part of the purchase accounting for the acquisition. For U.S. tax purposes, the future amortization of these intangibles will be non-deductible, thereby creating income. Since the Company files a consolidated U.S. tax return, the benefit from these identifiable intangible assets will be utilizable. The Company is required to determine its ability to use the tax benefit against the valuation allowance previously established. The Company has determined that it is more likely than not that these benefits will be recognized. As a result, the valuation allowance has been reduced for the assumed net deferred tax liabilities, resulting in an income tax benefit of $0.7 million. This benefit was included as a component of the Company's tax provision for the year ended December 31, 2018. In 2019, an adjustment of $0.2 million was recorded, reducing the income tax benefit from the Edgewater Acquisition to $0.5 million.

The 2017 acquisition of GENBAND was accounted for as a non-taxable business combination. GENBAND had previously been treated as a partnership for U.S. tax purposes. Consequently, U.S. federal and state deferred taxes were recorded as part of the business combination based on the differences between the tax basis of the acquired assets and assumed liabilities and their reported amounts for financial reporting purposes. The Company concluded that there was insufficient positive evidence to overcome the more objective negative evidence related to cumulated losses and other factors. The Company recorded a valuation allowance against the acquired deferred tax assets. The Company recorded identifiable intangible assets as part of the purchase accounting for the acquisition. For U.S. tax purposes, the future amortization of these intangibles will be non-deductible, thereby creating income. Since the Company files a consolidated U.S. tax return, the benefit from these identifiable intangible assets will be utilizable. The Company is required to determine its ability to use the tax benefit against the valuation allowance previously established. The Company has determined that it is more likely than not that these benefits will be recognized. As a result, the valuation allowance has been reduced for the assumed net deferred tax liabilities, resulting in an income tax benefit of $16.4 million. This benefit was included as a component of the Company's provision for income taxes for the year ended December 31, 2017.

MAJOR CUSTOMERS

12 Months Ended

Dec. 31, 2019

Risks and Uncertainties [Abstract]

MAJOR CUSTOMERS

The following customers contributed 10% or more of the Company's revenue in at least one of the years ended December 31, 2019, 2018 and 2017:



	Year ended December 31,
	2019	2018	2017
Verizon Communications Inc.	17%	17%	17%
AT&T Inc.	12%	*	*

* Less than 10% of total revenue.

At December 31, 2019, one customer accounted for 10% or more of the Company's accounts receivable balance, representing approximately 22% of total accounts receivable. At December 31, 2018, two customers accounted for 10% or more of the Company's accounts receivable balance, representing approximately 32% in the aggregate of total accounts receivable. The Company performs ongoing credit evaluations of its customers and generally does not require collateral on accounts receivable. The Company maintains an allowance for doubtful accounts and such losses have been within management's expectations.

RELATED PARTIES	12 Months Ended
RELATED PARTIES	Dec. 31, 2019
Related Party Transactions [Abstract]
RELATED PARTIES	RELATED PARTIES As a portion of the consideration for the Merger, on October 27, 2017, the Company issued a promissory note for $22.5 million to certain of GENBAND's equity holders who, following the Merger, owned greater than five percent of the Company's outstanding shares. As described in Note 12 above, the promissory note did not amortize and the principal thereon was payable in full on the third anniversary of its execution. Interest on the promissory note was payable quarterly in arrears and accrued at a rate of 7.5% per year for the first six months after issuance, and thereafter at a rate of 10% per year. The failure to make any payment under the promissory note when due and, with respect to payment of any interest, the continuation of such failure for a period of thirty days thereafter, constituted an event of default under the promissory note. If an event of default occurs under the promissory note, the payees could have declared the entire balance of the promissory note due and payable (including principal and accrued and unpaid interest) within five business days of the payees' notification to the Company of such acceleration. At December 31, 2018, the Promissory Note balance was $24.1 million, comprised of $22.5 million of principal, plus $1.6 million of interest converted to principal. On April 29, 2019, the Company repaid in full all outstanding amounts under the Promissory Note, aggregating $24.7 million. The Company did not incur any early termination penalties in connection with this repayment.

COMMITMENTS AND CONTINGENCIES	12 Months Ended
COMMITMENTS AND CONTINGENCIES	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Litigation As previously disclosed, the Company was involved in six lawsuits (together, the "Lawsuits") with Metaswitch Networks Ltd., Metaswitch Networks Corp. and Metaswitch Inc. (collectively, "Metaswitch"). In five of the Lawsuits, the Company was the plaintiff and, in three of those five lawsuits, the Company was also a counterclaim defendant. In the sixth case, the Company was the defendant. On April 22, 2019, the Company and Metaswitch agreed to a binding mediator's proposal that resolved the six Lawsuits between the Company and Metaswitch (the "Lawsuits"). The Company and Metaswitch signed a Settlement and Cross-License Agreement on May 29, 2019 (the "Royalty Agreement"). Pursuant to the terms of the Royalty Agreement, Metaswitch has agreed to pay the Company an aggregate amount of $63.0 million, which includes cash payments of $37.5 million during the second quarter of 2019 and $25.5 million payable in three installments annually, beginning June 26, 2020, with such installment payments accruing interest at a rate of 4% per year. As part of the Royalty Agreement, the Company and Metaswitch (i) have released the other from all claims and liabilities; (ii) have licensed each party's existing patent portfolio to the other party; and (iii) have requested the applicable courts to dismiss the Lawsuits. The Company received $37.5 million of aggregate payments from Metaswitch in the second quarter of 2019 and recorded notes receivable for future payments of $25.5 million, comprised of $8.5 million in Other current assets and $17.0 million in Other assets in the consolidated balance sheet at December 31, 2019. This activity is included in cash flows from operating activities in the consolidated statement of cash flows for the year ended December 31, 2019. The gain from the settlement of $63.0 million is included in Other income (expense), net, in the Company's consolidated statement of operations for the year ended December 31, 2019. Contingencies On November 8, 2018, Ron Miller, a purported stockholder of the Company, filed a Class Action Complaint (the "Miller Complaint") in the United States District Court for the District of Massachusetts (the "Massachusetts District Court") against the Company and three of its former officers (collectively, the "Defendants"), claiming to represent a class of purchasers of Sonus common stock during the period from January 8, 2015 through March 24, 2015 and alleging violations of the federal securities laws. Similar to a previous complaint entitled Sousa et al. vs. Sonus Networks, Inc. et al., which was dismissed with prejudice by an order dated June 6, 2017, the Miller Complaint claims that the Defendants made misleading forward-looking statements concerning Sonus' expected fiscal first quarter of 2015 financial performance, which statements were also the subject of an August 7, 2018 Securities and Exchange Commission Cease and Desist Order, whose findings the Company neither admitted nor denied. The Miller plaintiffs are seeking monetary damages. After the Miller Complaint was filed, several parties filed and briefed motions seeking to be selected by the Massachusetts District Court to serve as a Lead Plaintiff in the action. On June 21, 2019, the Massachusetts District Court appointed a group as Lead Plaintiffs and the Lead Plaintiffs filed an amended complaint on July 19, 2019. On August 30, 2019, the Defendants filed a motion to dismiss the Miller Complaint and, on October 4, 2019, the Lead Plaintiffs filed an opposition to the motion to dismiss. The Defendants filed a reply to such opposition on November 1, 2019. There was an oral argument on the motion to dismiss on February 12, 2020. In addition, the Company is often a party to disputes and legal proceedings that it considers routine and incidental to its business. Management does not expect the results of any of these actions to have a material effect on the Company's business or consolidated financial statements.

QUARTERLY RESULTS (UNAUDITED)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

QUARTERLY RESULTS (UNAUDITED)

The following tables present the Company's quarterly operating results for the years ended December 31, 2019 and 2018. The information for each of these quarters is unaudited and has been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, all necessary adjustments, consisting only of normal recurring adjustments, have been included to present fairly the unaudited consolidated quarterly results when read in conjunction with the Company's audited consolidated financial statements and related notes.



	First Quarter (1)			Second Quarter			Third Quarter		Fourth Quarter
	(In thousands, except per share data)
Fiscal 2019
Revenue	$	118,928		$	145,421		$	137,653	$	161,109
Cost of revenue	62,339			64,748			58,776		60,164
Gross profit	$	56,589		$	80,673		$	78,877	$	100,945
(Loss) income from operations	$	(36,228	)	$	(7,096	)	$	2,686	$	(148,822	)
Net (loss) income	$	(30,832	)	$	49,470		$	1,650	$	(150,363	)
(Loss) earnings per share (3):
Basic	$	(0.29	)	$	0.45		$	0.01	$	(1.36	)
Diluted	$	(0.29	)	$	0.45		$	0.01	$	(1.36	)
Shares used in computing (loss) earnings per share:
Basic	108,167			110,394			110,080		110,269
Diluted	108,167			110,698			110,756		110,269



	First Quarter			Second Quarter			Third Quarter (2)			Fourth Quarter
	(In thousands, except per share data)
Fiscal 2018
Revenue	$	121,180		$	137,361		$	152,468		$	166,896
Cost of revenue	65,907			62,250			70,234			71,182
Gross profit	$	55,273		$	75,111		$	82,234		$	95,714
Loss (income) from operations	$	(42,383	)	$	(16,636	)	$	(7,566	)	$	1,177
Net loss	$	(44,904	)	$	(19,922	)	$	(10,158	)	$	(1,826	)
Loss per share (3):
Basic	$	(0.44	)	$	(0.20	)	$	(0.10	)	$	(0.02	)
Diluted	$	(0.44	)	$	(0.20	)	$	(0.10	)	$	(0.02	)
Shares used in computing loss per share:
Basic	101,917			102,160			104,918			106,607
Diluted	101,917			102,160			104,918			106,607

__________________________________


(1)	Includes the results of Anova for the period subsequent to February 28, 2019.


(2)	Includes the results of Edgewater for the period subsequent to August 3, 2018.


(3)	(Loss) earnings per share is calculated independently for each of the quarters presented; accordingly, the sum of the quarterly (loss) earnings per share amounts may not equal the total calculated for the year.

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements have been prepared in United States dollars, in accordance with accounting principles generally accepted in the United States ("GAAP"). On February 28, 2019 (the "Anova Acquisition Date"), the Company acquired the business and technology assets of Anova Data, Inc. ("Anova"). The financial results of Anova are included in the Company's consolidated financial statements for the period subsequent to the Anova Acquisition Date. On August 3, 2018 (the "Edgewater Acquisition Date"), the Company completed the acquisition of Edgewater Networks, Inc. ("Edgewater" and such acquisition, the "Edgewater Acquisition"). The financial results of Edgewater are included in the Company's consolidated financial statements for the period subsequent to the Edgewater Acquisition Date. On October 27, 2017 (the "Merger Date"), Sonus Networks, Inc. ("Sonus") consummated an acquisition as specified in an Agreement and Plan of Merger (the “Merger Agreement”) with Solstice Sapphire Investments, Inc. ("NewCo") and certain of its wholly-owned subsidiaries, GENBAND Holdings Company, GENBAND Inc. and GENBAND II, Inc. (collectively, "GENBAND") pursuant to which, following a series of merger transactions (collectively, the "Merger"), Sonus and GENBAND each became a wholly-owned subsidiary of NewCo, with Sonus deemed the acquirer in the transaction for accounting purposes. Subsequently, on November 28, 2017, the Company changed its name from "Sonus Networks, Inc." to "Ribbon Communications Inc." The consolidated financial statements of the Company represent the consolidated financial statements of Sonus, prior to the Merger Date, and the consolidated financial statements of Ribbon, on and after the Merger Date. The financial results of GENBAND are included in Ribbon's consolidated financial statements beginning on the Merger Date.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of Ribbon and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates and Judgments	Use of Estimates and Judgments The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and judgments relied upon in preparing these consolidated financial statements include accounting for business combinations, revenue recognition for multiple element arrangements, inventory valuations, assumptions used to determine the fair value of stock-based compensation, intangible assets and goodwill valuations, legal contingencies and recoverability of Ribbon's net deferred tax assets and the related valuation allowances. Ribbon regularly assesses these estimates and records changes in estimates in the period in which they become known. Ribbon bases its estimates on historical experience and various other assumptions that it believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Reclassifications	Reclassifications Certain reclassifications, net affecting previously reported net loss, have been made to the previously issued financial statements to conform to the current period presentation.
Business Combinations	Business Combinations The Company recognizes identifiable assets acquired and liabilities assumed at their acquisition date fair values. Goodwill as of the acquisition date is measured as the excess of consideration transferred over the net of the acquisition date fair values of the assets acquired and the liabilities assumed and represents the expected future economic benefits arising from other assets acquired in the business combination that are not individually identified and separately recognized. While the Company uses its best estimates and assumptions as part of the purchase price allocation process to accurately value assets acquired and liabilities assumed at the acquisition date, its estimates are inherently uncertain and subject to refinement. As a result, during the measurement period, which may be up to one year from the acquisition date, the Company records adjustments to the assets acquired and liabilities assumed with the corresponding offset to goodwill to the extent that it identifies adjustments to the preliminary purchase price allocation. Upon the conclusion of the measurement period or final determination of the values of assets acquired and liabilities assumed, whichever comes first, any subsequent adjustments are recorded to the consolidated statements of operations.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers ("ASC 606" or the "New Revenue Standard") using the modified retrospective approach. As a result, the Company changed its accounting policy for revenue recognition, which is described below and in Note 14. The Company derives revenue from two primary sources: products and services. Product revenue includes the Company's appliances and software that function together to deliver the products' essential functionality. Software and appliances are also sold on a standalone basis. Services include customer support (software updates, upgrades and technical support), consulting, design services, installation services and training. Generally, contracts with customers contain multiple performance obligations, consisting of products and services. For these contracts, the Company accounts for individual performance obligations separately if they are considered distinct. When an arrangement contains more than one performance obligation, the Company will allocate the transaction price to each performance obligation on a relative standalone selling price basis. The Company utilizes the observable price of goods and services when they are sold separately to similar customers in order to estimate standalone selling price. The Company's software licenses typically provide a perpetual right to use the Company's software. The Company also sells term-based software licenses that expire and Software-as-a-Service ("SaaS")-based software which are referred to as subscription arrangements. The Company does not customize its software nor are installation services required, as the customer has a right to utilize internal resources or a third-party service company. The software and appliances are delivered before related services are provided and are functional without professional services or customer support. The Company has concluded that its software licenses are functional intellectual property that are distinct, as the user can benefit from the software on its own. The product revenue is typically recognized upon transfer of control or when the software is made available for download, as this is the point that the user of the software can direct the use of, and obtain substantially all of the remaining benefits from, the functional intellectual property. The Company does not recognize software revenue related to the renewal of subscription software licenses earlier than the beginning of the subscription period. Appliance products are generally sold with software to provide the customer solution. The Company offers warranties on its products. Certain of the Company's warranties are considered to be assurance-type in nature and do not cover anything beyond ensuring that the product is functioning as intended. Based on the guidance in ASC 606, assurance-type warranties do not represent separate performance obligations. The Company also sells separately-priced maintenance service contracts which qualify as service-type warranties and represent separate performance obligations. The Company does not allow and has no history of accepting product returns. Services revenue includes revenue from customer support and other professional services. Customer support includes software updates on a when-and-if-available basis, telephone support, integrated web-based support and bug fixes or patches. The Company sells its customer support contracts at a percentage of list or net product price related to the support. Customer support revenue is recognized ratably over the term of the customer support agreement, which is typically one year. The Company's professional services include consulting, technical support, resident engineer services, design services and installation services. Because control transfers over time, revenue is recognized based on progress toward completion of the performance obligation. The method to measure progress toward completion requires judgment and is based on the nature of the products or services to be provided. The Company generally uses the input method to measure progress for its contracts because it believes such method best depicts the transfer of assets to the customer, which occurs as the Company incurs costs for the contracts. Under the cost-to-cost measure of progress, the progress toward completion is measured based on the ratio of costs incurred to date to the total estimated costs at completion of the performance obligation. When the measure of progress is based upon expended labor, progress toward completion is measured as the ratio of labor time expended to date versus the total estimated labor time required to complete the performance obligation. Revenue is recorded proportionally as costs are incurred or as labor is expended. Costs to fulfill these obligations include internal labor as well as subcontractor costs. Customer training includes courses offered by the Company. The related revenue is typically recognized as the training services are performed.
Financial Instruments	Financial Instruments The carrying amounts of Ribbon's financial instruments approximate their fair values and include cash equivalents, investments, accounts receivable, borrowings under a revolving credit facility, accounts payable and long-term debt. All investments in marketable securities are classified as available-for-sale and are reported at fair value, with unrealized gains and losses excluded from earnings and reported, net of tax, in Accumulated other comprehensive income (loss), which is a component of stockholders' equity. Unrealized losses that are determined to be other-than-temporary, based on current and expected market conditions, are recognized in earnings. Declines in fair value determined to be credit-related are charged to earnings. The cost of marketable securities sold is determined by the specific identification method. Financial instruments with remaining maturities or that are due within one year from the balance sheet date are classified as current. Financial instruments with maturities or that are payable more than one year from the balance sheet date are classified as noncurrent.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash equivalents are stated at fair value. Cash equivalents are liquid securities that have remaining maturities of three months or less at the date of purchase.
Foreign Currency Translation	Foreign Currency Translation For foreign subsidiaries where the functional currency is the local currency, assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Revenue and expenses are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Accumulated other comprehensive income. For foreign subsidiaries where the functional currency is the U.S. dollar, monetary assets and liabilities are translated into U.S. dollars at the current exchange rate on the balance sheet date. Nonmonetary assets and liabilities are remeasured into U.S. dollars at historical exchange rates. Revenue and expense items are translated at average rates of exchange prevailing during each period. Translation adjustments for these subsidiaries are included in Other income (expense), net. Realized and unrealized foreign currency exchange gains and losses arising from transactions denominated in currencies other than the subsidiary's functional currency are reflected in earnings. Effective on the Merger Date, the Company began to record its foreign currency gains (losses) as a component of Other income (expense), net. The Company did not reclassify amounts previously recorded within General and administrative expenses as the amounts were not material to the consolidated results of the Company.
Inventory	Inventory Inventory is recorded at the lower of cost or market value using the first-in, first-out convention. The Company reduces the carrying value of inventory for those items that are potentially excess, obsolete or slow-moving based on changes in customer demand, technology developments or other economic factors. Ribbon writes down evaluation equipment at the time of shipment to its customers, as it is probable that the inventory value will not be realized. Deferred product costs represent deferred cost of revenue for product shipments to customers prior to satisfaction of Ribbon's revenue recognition criteria. The Company classifies inventory that is not expected to be consumed within one year from the balance sheet date as noncurrent and includes such inventory as a component of Other assets.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Expenditures for maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful lives of the related assets, which range from two to five years. Leasehold improvements are amortized over the lesser of the lease term or five years. When an asset is sold or retired, the cost and related accumulated depreciation or amortization are eliminated, and the resulting gain or loss, if any, is recognized in income (loss) from operations in the consolidated statement of operations. The Company reviews property and equipment for impairment in the same manner as intangible assets discussed below. Software development costs associated with internal use software are incurred in three stages of development: the preliminary project stage, the application development stage and the post-implementation stage. Costs incurred during the preliminary project and post-implementation stages are expensed as incurred. Certain qualifying costs incurred during the application development stage are capitalized as property and equipment. Internal use software is amortized on a straight-line basis over its estimated useful life of three years, beginning when the software is ready for its intended use.
Intangible Assets and Goodwill	Intangible Assets and Goodwill Intangible assets are primarily comprised of certain intangible assets arising from the Merger and the Edgewater Acquisition. These intangible assets include a combination of in-process research and development, developed technology, customer relationships, trade names, and internal use software. Intangible assets are reviewed for impairment when events or changes in circumstances indicate that their carrying amounts may not be recoverable based upon the estimated undiscounted cash flows. Recoverability of intangible assets with estimated lives and other long-lived assets is measured by a comparison of the carrying amount of an asset or asset group to future net undiscounted cash flows expected to be generated by the asset or asset group. If these comparisons indicate that an asset is not recoverable, the Company will recognize an impairment loss for the amount by which the carrying value of the asset or asset group exceeds the related estimated fair value. Estimated fair value is based on either discounted future operating cash flows or appraised values, depending on the nature of the asset. The Company amortizes its intangible assets over their respective useful lives, with the exception of in-process research and development, which has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology, and the Company begins to amortize this asset. See Note 9 for additional information regarding the Company's intangible assets. Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. Goodwill is not amortized, but instead is tested for impairment at least annually, or more frequently if indicators of potential impairment exist, by comparing the fair value of the Company's reporting unit to its carrying value. The Company's annual testing for impairment of goodwill is completed as of November 30. The Company operates as a single operating segment with one reporting unit and consequently evaluates goodwill for impairment based on an evaluation of the fair value of the Company as a whole. Based on the results of the Company's 2019 annual impairment test, the Company determined that its carrying value exceeded its fair value. The Company performed a fair value analysis using both an Income and Market approach which encompasses a discounted cash flow analysis and a guideline public company analysis using selected multiples. The Company recorded an impairment charge in the fourth quarter of 2019 of $164.3 million. The impairment charge is reported separately in the Company's consolidated statement of operations for the year ended December 31, 2019.
Stock-Based Compensation	Stock-Based Compensation The Company's stock-based compensation cost is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which generally represents the vesting period, and includes an estimate of the awards that will be forfeited. The Company uses the Black-Scholes valuation model for estimating the fair value on the date of grant of stock options. The fair value of stock option awards is affected by the Company's stock price as well as valuation assumptions, including the volatility of Ribbon's stock price, expected term of the option, risk-free interest rate and expected dividends. The Company may grant performance-based stock units ("PSUs") that include a market condition to certain of its executives. The Company uses a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the volatility of each entity and the pair-wise covariance between each entity. These results are then used to calculate the grant date fair values of the PSUs.
Concentrations of Credit Risk	Concentrations of Credit Risk The financial instruments that potentially subject Ribbon to concentrations of credit risk are cash, cash equivalents, investments and accounts receivable. The Company's cash equivalents and investments were managed by one financial institution at December 31, 2018. Historically, the Company has not experienced significant losses due to such bank depository concentration. Certain components and software licenses from third parties used in Ribbon's products are procured from single sources of supply. The failure of a supplier, including a subcontractor, to deliver on schedule could delay or interrupt Ribbon's delivery of products and thereby materially adversely affect Ribbon's revenue and operating results.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred and included as a component of Sales and marketing expense in the Company's consolidated statements of operations.
Operating Segments	Operating Segments The Company operates in a single segment, as the chief operating decision maker makes decisions and assesses performance at the company level. Operating segments are identified as components of an enterprise about which separate discrete financial information is utilized for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. To date, the chief operating decision maker has made such decisions and assessed performance at the company level, as one segment. The Company's chief operating decision makers are its Interim Co-Presidents and Chief Executive Officers.
Loss Contingencies and Reserves	Loss Contingencies and Reserves Ribbon is subject to ongoing business risks arising in the ordinary course of business, including legal claims, that affect the estimation process of the carrying value of assets, the recording of liabilities and the possibility of various loss contingencies. An estimated loss contingency is accrued when it is probable that a liability has been incurred or an asset has been impaired and the amount of loss can be reasonably estimated. Ribbon regularly evaluates current information available to determine whether such amounts should be adjusted and records changes in estimates in the period they become known. An allowance for doubtful accounts is estimated based on the Company's assessment of the collectability of specific customer accounts. Ribbon accrues for royalties for technology that it licenses from vendors based on established royalty rates and usage. Ribbon is periodically contacted by third parties who claim that Ribbon's products infringe on certain intellectual property of a third party. Ribbon evaluates these claims and accrues amounts when it is probable that the obligation has been incurred and the amounts are reasonably estimable.
Accounting for Leases	Accounting for Leases Effective January 1, 2019, the Company adopted the new standard on accounting for leases, ASC 842, Leases ("ASC 842"). ASC 842 replaced existing lease accounting rules with a comprehensive lease measurement and recognition standard and expanded disclosure requirements (see Note 17). ASC 842 requires lessees to recognize most leases on their balance sheets and eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. The Company elected to use the alternative transition method, which allowed entities to initially apply ASC 842 at the adoption date with no subsequent adjustments to prior period lease costs for comparability. The Company elected the package of practical expedients permitted under the transition guidance, which provided that a company need not reassess whether expired or existing contracts contained a lease, the lease classification of expired or existing leases, and the amount of initial direct costs for existing leases. In connection with the adoption of ASC 842, the Company recorded additional lease assets of $43.9 million and additional lease liabilities of $47.8 million as of January 1, 2019. The difference between the additional lease assets and lease liabilities, net of the deferred tax impact, was due to the absorption of related balances into the right-of-use assets, such as deferred rent. The adoption of this standard had no impact on the Company's consolidated statements of operations or cash flows.
Accounting for Income Taxes	Accounting for Income Taxes Deferred tax assets and liabilities are recognized for the expected future consequences of events that have been reflected in the consolidated financial statements. Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities and operating loss carryforwards, using tax rates expected to be in effect for the years in which the differences are expected to reverse. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. The Company has provided for income taxes on the undistributed earnings of its non-U.S. subsidiaries as of December 31, 2019, with the exception of the Company's Irish subsidiary, as the Company does not plan to permanently reinvest these amounts outside the United States. The repatriation of the undistributed earnings would result in withholding taxes imposed on the repatriation. Consequently, the Company has recorded a tax liability of $4.8 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2019. Had the earnings of the Irish subsidiary been determined to not be permanently reinvested outside the U.S., no additional deferred tax liability would be required due to no withholding taxes or income tax expense being imposed on such repatriation. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of the benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes.
Recent Accounting Pronouncements	Recent Accounting Pronouncements The Financial Accounting Standards Board ("FASB") has issued the following accounting pronouncements, all of which became effective for the Company on January 1, 2019 and none of which had a material impact on the Company's consolidated financial statements: In July 2018, the FASB issued Accounting Standards Update ("ASU") 2018-09, Codification Improvements (“ASU 2018-09”), which contains amendments to clarify, correct errors in or make minor improvements to the FASB codification. ASU 2018-09 makes improvements to multiple topics, including but not limited to comprehensive income, debt, income taxes related to both stock-based compensation and business combinations, fair value measurement and defined contribution benefit plans. In June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which expands the scope of ASC 718, Compensation - Stock Compensation ("ASC 718"), to include all share-based payment arrangements related to the acquisition of goods and services from both nonemployees and employees. As a result, most of the guidance in ASC 718 associated with employee share-based payments, including most of its requirements related to classification and measurement, applies to nonemployee share-based payment arrangements. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which amends ASC 220, Income Statement - Reporting Comprehensive Income, to allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (the "Tax Act") and requires entities to provide certain disclosures regarding stranded tax effects. The Company did not elect to reclassify the income tax effects of the Tax Act from accumulated other comprehensive income to accumulated deficit. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory, which removes the prohibition in ASC 740, Income Taxes, against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. In addition, the FASB has issued the following accounting pronouncements, none of which the Company believes will have a material impact on its consolidated financial statements: In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (“ASU 2018-15”), which provides guidance on implementation costs incurred in a cloud computing arrangement (“CCA”) that is a service contract. ASU 2018-15 amends ASC 350, Intangibles - Goodwill and Other (“ASC 350”) to include in its scope implementation costs of a CCA that is a service contract and clarifies that a customer should apply the guidance in ASC 350-40 to determine which implementation costs should be capitalized in such a CCA. ASU 2018-15 was effective for the Company beginning January 1, 2020. In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20): Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans (“ASU 2018-14”), which amends ASC 715, Compensation - Retirement Benefits, to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. ASU 2018-14 was effective for the Company beginning January 1, 2020. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which changes the fair value measurement requirements of ASC 820, Fair Value Measurement. ASU 2018-13 was effective for the Company beginning January 1, 2020 for both interim and annual reporting. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"), which adds an impairment model that is based on expected losses rather than incurred losses. Under ASU 2016-13, an entity recognizes as an allowance its estimate of expected credit losses, which the FASB believes will result in more timely recognition of such losses. In April and May 2019, the FASB issued ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments - Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments ("ASU 2019-04") and ASU 2019-05 Financial Instruments - Credit Losses (Topic 326): Targeted Transition Relief ("ASU 2019-05"), respectively. ASU 2019-04 provides transition relief for entities adopting ASU 2016-13 and ASU 2019-05 clarifies certain aspects of the accounting for credit losses, hedging activities and financial instruments in connection with the adoption of ASU 2016-13. ASU 2019-04 and ASU 2019-05 are effective with the adoption of ASU 2016-13, which was effective for the Company beginning January 1, 2020 for both interim and annual reporting periods.

EARNINGS (LOSS) PER SHARE (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Schedule of calculations of shares used to compute basic and diluted earnings (loss) per share

The calculations of shares used to compute basic and diluted loss per share are as follows (in thousands):



	Year ended December 31,
	2019	2018	2017
Weighted average shares outstanding—basic	109,734	103,916	58,822
Potential dilutive common shares	—	—	—
Weighted average shares outstanding—diluted	109,734	103,916	58,822

BUSINESS ACQUISITONS (Tables)

12 Months Ended

Dec. 31, 2019

Business Acquisition [Line Items]

Schedule of Components of Acquisition Related Costs

The components of acquisition- and integration-related costs incurred in the years ended December 31, 2019, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Professional and services fees (acquisition-related)	$	8,657	$	7,627	$	11,916
Management bonuses (acquisition-related)	—		1,972		931
Integration-related expenses	4,296		7,352		1,916
	$	12,953	$	16,951	$	14,763

Edgewater

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the allocation of the purchase consideration for Edgewater is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Cash paid to Edgewater Selling Stakeholders	$	51,162
Less cash acquired	(4,773		)
Net cash consideration	46,389
Unpaid cash consideration	30,000
Fair value of Ribbon stock issued	30,000
Fair value of equity awards assumed (see Note 16)	747
Fair value of total consideration	$	107,136

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	16,098
Property and equipment	245
Intangible assets:
Developed technology	29,500
Customer relationships	26,100
Trade names	1,100
Goodwill	48,053
Other noncurrent assets	103
Deferred revenue	(2,749		)
Other current liabilities	(9,926		)
Deferred revenue, net of current	(669		)
Other long-term liabilities	(719		)
	$	107,136

GENBAND

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the final allocation of the purchase consideration for GENBAND is as follows (in thousands):



Fair value of consideration transferred:
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	$	47,973
Payment of GENBAND management fees due to majority shareholder	10,302
Less cash acquired	(15,324		)
Net cash consideration	42,951
Fair value of Sonus stock issued	413,982
Promissory note issued to GENBAND equity holders	22,500
Fair value of total consideration	$	479,433

Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	$	99,126
Property and equipment	16,770
Intangible assets:
In-process research and development	5,600
Developed technology	129,000
Customer relationships	101,300
Trade names	900
Goodwill	285,825
Other noncurrent assets	6,732
Revolving credit facility	(17,930		)
Deferred revenue	(32,390		)
Other current liabilities	(80,023		)
Deferred revenue, net of current	(6,804		)
Other long-term liabilities	(28,673		)
	$	479,433

Business Acquisition, Pro Forma Information

Accordingly, these unaudited pro forma results are presented for illustrative purposes and are not intended to represent or be indicative of the actual results of operations of the combined company that would have been achieved had the Merger occurred at the beginning of the periods presented, nor are they intended to represent or be indicative of future results of operations (in thousands, except per share amounts):



	Year ended December 31,
	2017
	(unaudited)
Revenue	$	615,286
Net loss	$	(69,741	)
Loss per share	$	(0.69	)

CASH EQUIVALENTS AND INVESTMENTS (Tables)

12 Months Ended

Dec. 31, 2019

Investments, All Other Investments [Abstract]

Schedule of amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments

The amortized cost, gross unrealized gains and losses and fair value of the Company's cash equivalents and marketable securities at December 31, 2018 were comprised of the following (in thousands):



	December 31, 2018
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	310	$	—	$	—		$	310

Marketable securities
U.S. government agency notes	$	3,998	$	—	$	(9	)	$	3,989
Corporate debt securities	3,301		—		(6		)	3,295
	$	7,299	$	—	$	(15	)	$	7,284

Schedule of fair value of financial assets



			Fair value measurements at December 31, 2018 using:
	Total carrying value at December 31, 2018		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	310	$	310	$	—	$	—

Marketable securities
U.S. government agency notes	$	3,989	$	—	$	3,989	$	—
Corporate debt securities	3,295		—		3,295		—
	$	7,284	$	—	$	7,284	$	—

ACCOUNTS RECEIVABLE, NET (Tables)

12 Months Ended

Dec. 31, 2019

Receivables [Abstract]

Schedule of accounts receivable, net

Accounts receivable, net, consisted of the following (in thousands):



	December 31,
	2019			2018
Accounts receivable	$	193,619		$	188,522
Allowance for doubtful accounts	(913		)	(669		)
Accounts receivable, net	$	192,706		$	187,853

Schedule of allowance for doubtful accounts

The Company's allowance for doubtful accounts activity was as follows (in thousands):



Year ended December 31,	Balance at beginning of year		Charges to expense		Charges (credits) to other accounts (deferred revenue)			Write-offs			Balance at end of year
2019	$	669	$	738	$	68		$	(562	)	$	913
2018	$	73	$	351	$	620		$	(375	)	$	669
2017	$	10	$	154	$	(56	)	$	(35	)	$	73

INVENTORY (Tables)

12 Months Ended

Dec. 31, 2019

Inventory Disclosure [Abstract]

Schedule of inventory

Inventory consisted of the following (in thousands):



	December 31,
	2019			2018
On-hand final assemblies and finished goods inventories	$	13,283		$	19,879
Deferred cost of goods sold	2,441			3,798
	15,724			23,677
Less noncurrent portion (included in Other assets)	(924		)	(1,075		)
Current portion	$	14,800		$	22,602

PROPERTY AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2019

Property, Plant and Equipment [Abstract]

Schedule of property and equipment

Property and equipment consisted of the following (in thousands):



		December 31,
	Useful Life	2019			2018
Equipment	2-5 years	$	82,737		$	76,423
Software	2-5 years	27,939			24,707
Furniture and fixtures	3-5 years	1,283			1,490
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	23,975			21,220
		135,934			123,840
Less accumulated depreciation and amortization		(106,958		)	(96,798		)
Property and equipment, net		$	28,976		$	27,042

Property and equipment under finance leases included in the amounts above were as follows (in thousands):



	December 31,
	2019			2018
Cost	$	4,401		$	2,979
Less accumulated depreciation	(1,981		)	(875		)
Property and equipment under finance leases, net	$	2,420		$	2,104

The net book values of the Company's property and equipment by geographic area were as follows (in thousands):



	December 31,
	2019		2018
United States	$	17,584	$	17,862
Canada	4,768		4,076
Asia/Pacific	5,146		3,841
Europe	1,224		1,100
Other	254		163
	$	28,976	$	27,042

INTANGIBLE ASSETS AND GOODWILL (Tables)

12 Months Ended

Dec. 31, 2019

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of intangible assets

The Company's intangible assets at December 31, 2019 and 2018 consisted of the following (in thousands):



December 31, 2019	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.79	188,880		100,760		88,120
Customer relationships	9.46	152,140		33,350		118,790
Trade names	5.20	2,000		1,144		856
Internal use software	3.00	730		730		—
	7.82	$	349,350	$	135,984	$	213,366



December 31, 2018	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.91	182,880		63,187		119,693
Customer relationships	9.44	146,940		22,218		124,722
Trade names	5.20	2,000		624		1,376
Internal use software	3.00	730		730		—
	7.88	$	338,150	$	86,759	$	251,391

* An in-process research and development intangible asset has an indefinite life until the product is generally available, at which time such asset is typically reclassified to developed technology.

Schedule of amortization expense related to intangible assets

Amortization expense for intangible assets for the years ended December 31, 2019, 2018 and 2017 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2019		2018		2017		Statement of operations classification
Developed technology	$	37,573	$	38,976	$	18,358	Cost of revenue - product
Customer relationships	11,132		10,203		4,145		Sales and marketing
Trade names	520		544		80		Sales and marketing
	$	49,225	$	49,723	$	22,583

Schedule of estimated future amortization expense for intangible assets

Estimated future amortization expense for the Company's intangible assets at December 31, 2019 was as follows (in thousands):



Years ending December 31,
2020	$	48,952
2021	43,495
2022	35,092
2023	27,271
2024	20,201
Thereafter	38,355
	$	213,366

Schedule of goodwill

The changes in the carrying value of the Company's goodwill in the years ended December 31, 2019 and 2018 were as follows (in thousands):



	Year ended December 31,
	2019			2018
Balance at January 1
Goodwill	$	386,761		$	338,822
Accumulated impairment losses	(3,106		)	(3,106		)
	383,655			335,716
Acquisition of Anova	5,541			—
Acquisition of Edgewater	—			48,053
Write-off of goodwill attributable to dissolved subsidiary	—			(114		)
Impairment of goodwill	(164,300		)	—
Balance at December 31	$	224,896		$	383,655

The components of goodwill at December 31, 2019 and 2018 were as follows (in thousands):




	December 31,
	2019			2018
Goodwill	$	392,302		$	386,761
Accumulated impairment losses	(167,406		)	(3,106		)
	$	224,896		$	383,655

ACCRUED EXPENSES AND OTHER (Tables)

12 Months Ended

Dec. 31, 2019

Payables and Accruals [Abstract]

Schedule of accrued expenses

Accrued expenses and other consisted of the following (in thousands):



	December 31,
	2019		2018
Employee compensation and related costs	$	27,166	$	42,852
Professional fees	13,331		7,994
Deferred purchase consideration - business acquisitions	1,700		15,000
Other	14,503		18,417
	$	56,700	$	84,263

RESTRUCTURING AND FACILITIES CONSOLIDATION INTIATIVES (Tables)

12 Months Ended

Dec. 31, 2019

Restructuring and Related Activities [Abstract]

Summary of restructuring accrual activity

Summaries of the Merger Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Initiatives charged to expense		Adjustment for the impact of ASC 842 adoption			Cash payments			Balance at December 31, 2019
Severance	$	1,910	$	5,076	$	—		$	(6,577	)	$	409
Facilities	771		156		(771		)	(156		)	—
	$	2,681	$	5,232	$	(771	)	$	(6,733	)	$	409



Year ended December 31, 2018	Balance at January 1, 2018		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2018
Severance	$	7,595	$	14,735	$	(5	)	$	(20,415	)	$	1,910
Facilities	—		1,399		—			(628		)	771
	$	7,595	$	16,134	$	(5	)	$	(21,043	)	$	2,681

Summaries of the GENBAND Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Adjustment for the impact of ASC 842 adoption			Balance at December 31, 2019
Facilities	$	117	$	(117	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2018
Severance	$	1,916	$	487	$	(2,403	)	$	—
Facilities	205		399		(487		)	117
	$	2,121	$	886	$	(2,890	)	$	117

Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Cash payments			Balance at December 31, 2019
Facilities	$	58	$	(58	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Cash payments			Balance at December 31, 2018
Facilities	$	95	$	(37	)	$	58

Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2019 and 2018 are as follows (in thousands):



Year ended December 31, 2019	Balance at January 1, 2019		Adjustment for the impact of ASC 842 adoption			Balance at December 31, 2019
Facilities	$	33	$	(33	)	$	—



Year ended December 31, 2018	Balance at January 1, 2018		Cash payments			Balance at December 31, 2018
Facilities	$	365	$	(332	)	$	33

A summary of the 2019 Restructuring Initiative accrual activity, excluding the accelerated amortization of lease assets, for the year ended December 31, 2019 is as follows (in thousands):



	Balance at January 1, 2019		Initiatives charged to expense		Cash payments			Balance at December 31, 2019
Severance	$	—	$	6,103	$	(3,993	)	2,110
Facilities	—		1,372		(381		)	991
	$	—	$	7,475	$	(4,374	)	$	3,101

The components of Restructuring and related expense recorded in the years ended December 31, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2018		2017
Severance and related costs	15,217		8,925
Facilities	1,798		511
	$	17,015	$	9,436

The components of restructuring and related expense for the year ended December 31, 2019 were as follows (in thousands):



Severance and related costs	$	11,179
Variable and other facilities-related costs	1,528
Accelerated amortization of lease assets due to cease-use	3,692
	$	16,399

LONG-TERM LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2019

Other Liabilities [Abstract]

Schedule of long-term liabilities

Long-term liabilities consisted of the following (in thousands):



	December 31,
	2019			2018
Finance lease obligations	$	3,149		$	2,363
Deferred rent	—			3,039
Restructuring	3,510			979
Pension obligations	9,954			7,006
Taxes payable	1,991			1,818
Deferred purchase consideration	—			30,000
Other	1,683			2,425
	20,287			47,630
Current portion	(3,698		)	(16,833		)
Long-term liabilities, net of current portion	$	16,589		$	30,797

REVENUE RECOGNITION (Tables)

12 Months Ended

Dec. 31, 2019

Revenue from Contract with Customer [Abstract]

Schedule of Timing of Performance Obligation

The Company's typical performance obligations include the following:



Performance Obligation	When Performance Obligation is Typically Satisfied	When Payment is Typically Due
Software and Product Revenue
Software licenses (perpetual or term)	Upon transfer of control; typically, when made available for download (point in time)	Generally, within 30 days of invoicing except for term licenses, which may be paid for over time

Software licenses (subscription)	Upon activation of hosted site (over time)	Generally, within 30 days of invoicing

Appliances	When control of the appliance passes to the customer; typically, upon delivery (point in time)	Generally, within 30 days of invoicing

Software upgrades	Upon transfer of control; typically, when made available for download (point in time)	Generally, within 30 days of invoicing

Customer Support Revenue
Customer support	Ratably over the course of the support contract (over time)	Generally, within 30 days of invoicing

Professional Services
Other professional services (excluding training services)	As work is performed (over time)	Generally, within 30 days of invoicing (upon completion of services)

Training	When the class is taught (point in time)	Generally, within 30 days of services being performed

Disaggregation of Revenue

The Company's total revenue for the years ended December 31, 2019, 2018 and 2017 was disaggregated geographically as follows:



Year ended December 31, 2019	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	170,937	$	133,271	$	37,085	$	341,293
Europe, Middle East and Africa	42,262		43,186		12,279		97,727
Japan	13,065		11,692		5,842		30,599
Other Asia Pacific	17,552		16,106		4,879		38,537
Other	18,214		29,973		6,768		54,955
	$	262,030	$	234,228	$	66,853	$	563,111



Year ended December 31, 2018	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	169,510	$	132,282	$	35,832	$	337,624
Europe, Middle East and Africa	37,833		46,856		11,794		96,483
Japan	23,108		11,234		5,069		39,411
Other Asia Pacific	30,575		12,321		4,358		47,254
Other	17,988		31,273		7,872		57,133
	$	279,014	$	233,966	$	64,925	$	577,905



Year ended December 31, 2017	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	121,121	$	75,040	$	22,896	$	219,057
Europe, Middle East and Africa	23,352		17,471		3,742		44,565
Japan	10,252		10,282		3,855		24,389
Other Asia Pacific	14,693		5,901		1,952		22,546
Other	11,701		6,041		1,643		19,385
	$	181,119	$	114,735	$	34,088	$	329,942



	Year ended December 31,
	2019		2018		2017
Indirect sales through channel program	$	94,639	$	69,232	$	43,138
Direct sales	167,391		209,782		137,981
	$	262,030	$	279,014	$	181,119

The Company's product revenue from sales to enterprise customers and from sales to service provider customers for the years ended December 31, 2019, 2018 and 2017 was as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Sales to enterprise customers	$	70,548	$	57,534	$	35,592
Sales to service provider customers	191,482		221,480		145,527
	$	262,030	$	279,014	$	181,119

Schedule of Customer Assets and Liabilities

The changes in the Company's accounts receivable, unbilled receivables and deferred revenue balances for the years ended December 31, 2019 and 2018 were as follows (in thousands):



	Accounts receivable			Unbilled accounts receivable		Deferred revenue (current)			Deferred revenue (long-term)
Balance at January 1, 2019	$	174,310		$	13,543	$	105,087		$	17,572
Increase (decrease), net	(5,808		)	10,661		(4,681		)	2,910
Balance at December 31, 2019	$	168,502		$	24,204	$	100,406		$	20,482



	Accounts receivable		Unbilled accounts receivable			Deferred revenue (current)		Deferred revenue (long-term)
Balance at January 1, 2018	$	149,122	$	16,034		$	100,571	$	14,184
Increase (decrease), net	25,188		(2,491		)	4,516		3,388
Balance at December 31, 2018	$	174,310	$	13,543		$	105,087	$	17,572

STOCK-BASED COMPENSATION PLANS (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Schedule of activity related to outstanding stock options

The activity related to the Company's outstanding stock options during the year ended December 31, 2019 was as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2019	582,061		$	9.01
Granted	—		$	—
Exercised	(127,334	)	$	1.85
Forfeited	(38,666	)	$	2.55
Expired	(118,937	)	$	12.43
Outstanding at December 31, 2019	297,124		$	11.55	4.95	$	122
Vested or expected to vest at December 31, 2019	293,782		$	11.65	4.92	$	119
Exercisable at December 31, 2019	260,152		$	12.89	4.59	$	85

Schedule of assumptions used to estimate the fair value of options at the date of grant using the Black-Scholes option pricing model

The grant date fair values of stock options granted in the year ended December 31, 2017 were estimated using the Black-Scholes valuation model with the following assumptions:



Risk-free interest rate		1.22% - 1.95%
Expected dividends		—
Weighted average volatility		51.1%
Expected life (years)		5.0

Schedule of activity related to unvested restricted stock grants

The activity related to the Company's RSAs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	1,508,011		$	6.90
Granted	—		$	—
Vested	(967,291	)	$	6.90
Forfeited	(52,744	)	$	7.04
Unvested balance at December 31, 2019	487,976		$	6.87

The activity related to the Company's RSUs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	636,300		$	6.52
Granted	2,828,832		$	4.98
Vested	(537,416	)	$	6.04
Forfeited	(137,656	)	$	5.35
Unvested balance at December 31, 2019	2,790,060		$	5.11

Schedule of activity related to performance stock awards

The activity related to the Company's PSUs for the year ended December 31, 2019 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2019	210,416		$	5.77
Granted	872,073		$	6.03
Vested	(9,466	)	$	8.55
Forfeited	(5,950	)	$	8.55
Unvested balance at December 31, 2019	1,067,073		$	5.94

Schedule of stock-based compensation expenses which are included in condensed consolidated statement of operations

The consolidated statements of operations included stock-based compensation for the years ended December 31, 2019, 2018 and 2017 as follows (in thousands):



	Year ended December 31,
	2019		2018		2017
Product cost of revenue	$	76	$	114	$	514
Service cost of revenue	478		345		1,448
Research and development	1,898		1,797		7,337
Sales and marketing	3,028		2,935		4,885
General and administrative	7,121		5,881		11,473
	$	12,601	$	11,072	$	25,657

Schedule of common stock reserved for future issuance

Common stock reserved for future issuance at December 31, 2019 consists of the following:



2019 Plan	7,051,559
ESPP	1,148,867
	8,200,426

LEASES (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Right-of-use Lease Assets and Lease Liabilities

The Company's right-of-use lease assets and lease liabilities at December 31, 2019 and December 31, 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Assets
Operating lease assets	$	36,654	$	—
Finance lease assets*	2,420		2,104
Total leased assets	$	39,074	$	2,104

Liabilities
Current
Operating	$	7,719	$	—
Finance	1,005		1,039
Noncurrent
Operating	37,202		—
Finance	2,144		1,324
Total lease liabilities	$	48,070	$	2,363

Lease, Other Information

The components of lease expense for the year ended December 31, 2019 were as follows (in thousands):




Operating lease cost*	$	13,865
Finance lease cost
Amortization of leased assets	1,106
Interest on lease liabilities	265
Short-term lease cost	19,460
Variable lease costs (costs excluded from minimum fixed lease payments)**	3,264
Sublease income	(374		)
Net lease cost	$	37,586

Other information related to the Company's leases as of and for the year ended December 31, 2019 was as follows (in thousands, except lease terms and percentages):



Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	10,559
Operating cash flows from finance leases	$	265
Financing cash flows from finance leases	$	913
Weighted average remaining lease term (years):
Operating leases	6.73
Finance leases	2.35
Weighted average discount rate:
Operating leases	6.50		%
Finance leases	7.54		%

Schedule of Future Minimum Fixed Operating Lease Payments

Future minimum fixed lease payments under noncancelable leases at December 31, 2019 were as follows (in thousands):



	Operating			Finance
	leases			leases
2020	$	10,290		$	1,644
2021	9,468			1,159
2022	7,665			581
2023	7,067			—
2024	5,303			—
2025 and beyond	15,738			—
Total lease payments	55,531			3,384
Less: interest	(10,610		)	(235		)
Present value of lease liabilities	$	44,921		$	3,149

Schedule of Future Minimum Fixed Finance Lease Payments

Future minimum fixed lease payments under noncancelable leases at December 31, 2019 were as follows (in thousands):



	Operating			Finance
	leases			leases
2020	$	10,290		$	1,644
2021	9,468			1,159
2022	7,665			581
2023	7,067			—
2024	5,303			—
2025 and beyond	15,738			—
Total lease payments	55,531			3,384
Less: interest	(10,610		)	(235		)
Present value of lease liabilities	$	44,921		$	3,149

Schedule of Future Minimum Fixed Lease Payments, Operating

Future minimum fixed lease payments under noncancelable leases at December 31, 2018 were as follows (in thousands):



	Operating		Finance
	leases*		leases
2019	$	10,705	$	1,386
2020	8,384		1,010
2021	7,455		288
2022	5,691		—
2023	5,430		—
2024 and beyond	19,818		—
Total lease payments	$	57,483	2,684
Less: interest			(321		)
Present value of lease liabilities**			$	2,363

Schedule of Future Minimum Fixed Lease Payments, Finance

Future minimum fixed lease payments under noncancelable leases at December 31, 2018 were as follows (in thousands):



	Operating		Finance
	leases*		leases
2019	$	10,705	$	1,386
2020	8,384		1,010
2021	7,455		288
2022	5,691		—
2023	5,430		—
2024 and beyond	19,818		—
Total lease payments	$	57,483	2,684
Less: interest			(321		)
Present value of lease liabilities**			$	2,363

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS (Tables)

12 Months Ended

Dec. 31, 2019

Retirement Benefits [Abstract]

Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan



	Year ended December 31, 2019			Year ended December 31, 2018
Changes in projected benefit obligations:
Projected benefit obligation, beginning of year	$	10,848		$	11,484
Service cost	335			449
Interest cost	140			150
Participant contributions	24			5
Benefits and expenses paid	(44		)	(23		)
Net actuarial loss (gain) on obligation	1,059			(414		)
Curtailment	82			(553		)
Settlement	(660		)	(250		)
Projected benefit obligation, end of year	$	11,784		$	10,848

Changes in plan assets:
Fair value of plan assets, beginning of year	$	3,842		$	3,893
Actual return on plan assets	(1,471		)	(53		)
Employer contributions	139			292
Participant contributions	24			5
Administrative expenses	(21		)	(22		)
Benefits paid	(683		)	(273		)
Fair value of plan assets, end of year	$	1,830		$	3,842

Funded status at end of year	$	(9,954	)	$	(7,006	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	2,743		$	222

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(74	)	$	(75	)
Other long-term liabilities (non-current pension liability)	(9,880		)	(6,931		)
Net amount recognized	$	(9,954	)	$	(7,006	)

Schedule of Accumulated Benefit Obligations

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2019 and 2018 were as follows (in thousands):



	December 31, 2019		December 31, 2018
Aggregate projected benefit obligation	$	11,784	$	10,848
Aggregate accumulated benefit obligation	$	7,759	$	7,152
Aggregate fair value of plan assets	$	1,830	$	3,842

Schedule of Net Benefit Costs

Net periodic benefit costs for the years ended December 31, 2019 and 2018 and the period from the Merger Date to December 31, 2017 were as follows (in thousands):



	Year ended December 31, 2019			Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	335		$	449		$	68
Interest cost	140			150			25
Expected return on plan assets	(14		)	(45		)	(8		)
Plan asset expenses	21			22			4
Curtailment charge (credit)	13			(510		)	—
Settlement charge	115			3			—
Net periodic benefit costs	$	610		$	69		$	89

Schedule of Expected Benefit Payments

Expected benefit payments for the next ten years are as follows (in thousands):



Years ending December 31,
2020	$	74
2021	94
2022	45
2023	223
2024	57
2025 to 2029	1,661
	$	2,154

Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss)



	Year ended December 31, 2019		Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Net loss (gain)	$	2,526	$	(356	)	$	578

Schedule of Assumptions Used

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2019 and 2018 were as follows:



	December 31, 2019		December 31, 2018
Discount rate	0.68	%	1.30	%
Rate of compensation increase	2.88	%	2.83	%



	Year ended December 31, 2019		Year ended December 31, 2018		October 27, 2017 to December 31, 2017
Discount rate	1.30	%	1.50	%	1.49	%
Expected long-term return on plan assets	1.12	%	1.34	%	1.23	%
Rate of compensation increase	2.83	%	3.38	%	3.38	%

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Income (Loss) Before Taxes

The components of loss from continuing operations before income taxes consisted of the following (in thousands):



	Year ended December 31,
	2019			2018			2017
Income (loss) before income taxes:
United States	$	(132,887	)	$	(52,569	)	$	(55,932	)
Foreign	9,994			(20,841		)	2,240
	$	(122,893	)	$	(73,410	)	$	(53,692	)

Schedule of Income Tax Expense (Benefit)

The provision (benefit) for income taxes from continuing operations consisted of the following (in thousands):



	Year ended December 31,
	2019			2018			2017
Provision (benefit) for income taxes:
Current:
Federal	$	11		$	561		$	(200	)
State	128			128			115
Foreign	1,744			2,198			1,960
Total current	1,883			2,887			1,875
Deferred:
Federal	9,790			(8,481		)	49,570
State	1,630			(1,414		)	(4,833		)
Foreign	383			(1,477		)	(816		)
Change in valuation allowance	(6,504		)	11,885			(64,236		)
Total deferred	5,299			513			(20,315		)
Total	$	7,182		$	3,400		$	(18,440	)

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the Company's effective tax rate for continuing operations to the statutory federal rate is as follows:



	Year ended December 31,
	2019		2018		2017
U.S. statutory income tax rate	21.0	%	21.0	%	35.0	%
State income taxes, net of federal benefit	(0.2	)	(0.1	)	1.2
Foreign income taxes	(1.0	)	(5.7	)	(0.5	)
Acquisition costs	(0.5	)	(0.3	)	(6.0	)
Foreign deemed dividends	(0.4	)	(3.4	)	(3.8	)
Stock-based compensation	(0.7	)	(0.3	)	26.8
Tax credits	2.8		0.6		(33.3	)
Uncertain tax positions	(0.2	)	1.3		(1.2	)
NOL and credit limitations	—		—		(18.9	)
Valuation allowance	(0.7	)	(16.1	)	29.0
Goodwill amortization	0.4		0.3		(1.7	)
Meals and entertainment	(0.3	)	(0.4	)	(0.5	)
Tax reform	(0.1	)	—		8.8
Goodwill impairment	(25.4	)	—		—
Other, net	(0.5	)	(1.5	)	(0.6	)
Effective income tax rate	(5.8	)%	(4.6	)%	34.3	%

Summary of Deferred Tax Assets and Liabilities

The following is a summary of the significant components of deferred income tax assets and liabilities (in thousands):



	December 31,
	2019			2018
Assets:
Net operating loss carryforwards	$	61,057		$	76,278
Research and development tax credits	32,879			28,664
Deferred revenue	7,868			5,755
Accrued expenses	5,687			9,601
Inventory	4,618			4,906
Stock-based compensation	2,880			1,536
Fixed assets	5,461			7,716
Other temporary differences	2,138			1,943
	122,588			136,399
Valuation allowance	(94,980		)	(101,484		)
Total deferred tax assets	27,608			34,915
Liabilities:
Purchased intangible assets	(22,470		)	(26,014		)
Unremitted foreign income	(4,827		)	(4,487		)
Total deferred tax liabilities	(27,297		)	(30,501		)
Total net deferred tax assets	$	311		$	4,414

The deferred tax assets and liabilities based on tax jurisdictions are presented in the Company's consolidated balance sheets as follows:
Deferred income taxes - noncurrent assets	$	4,959		$	9,152
Deferred income taxes - noncurrent liabilities	(4,648		)	(4,738		)
	$	311		$	4,414

Schedule of Unrecognized Tax Benefits

A reconciliation of the Company's unrecognized tax benefits is as follows (in thousands):



	2019			2018			2017
Unrecognized tax benefits at January 1	$	3,461		$	4,528		$	8,969
Increases related to current year tax positions	292			74			139
Increases related to prior period tax positions	—			122			430
Increases related to business acquisitions	—			—			2,012
Decreases related to prior period tax positions	(821		)	(1,263		)	(7,022		)
Settlements	—			—			—
Unrecognized tax benefits at December 31	$	2,932		$	3,461		$	4,528

MAJOR CUSTOMERS (Tables)

12 Months Ended

Dec. 31, 2019

Risks and Uncertainties [Abstract]

Schedule of customers contributing 10% or more of the revenue

The following customers contributed 10% or more of the Company's revenue in at least one of the years ended December 31, 2019, 2018 and 2017:



	Year ended December 31,
	2019	2018	2017
Verizon Communications Inc.	17%	17%	17%
AT&T Inc.	12%	*	*

* Less than 10% of total revenue.

QUARTERLY RESULTS (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly financial information

In the opinion of management, all necessary adjustments, consisting only of normal recurring adjustments, have been included to present fairly the unaudited consolidated quarterly results when read in conjunction with the Company's audited consolidated financial statements and related notes.



	First Quarter (1)			Second Quarter			Third Quarter		Fourth Quarter
	(In thousands, except per share data)
Fiscal 2019
Revenue	$	118,928		$	145,421		$	137,653	$	161,109
Cost of revenue	62,339			64,748			58,776		60,164
Gross profit	$	56,589		$	80,673		$	78,877	$	100,945
(Loss) income from operations	$	(36,228	)	$	(7,096	)	$	2,686	$	(148,822	)
Net (loss) income	$	(30,832	)	$	49,470		$	1,650	$	(150,363	)
(Loss) earnings per share (3):
Basic	$	(0.29	)	$	0.45		$	0.01	$	(1.36	)
Diluted	$	(0.29	)	$	0.45		$	0.01	$	(1.36	)
Shares used in computing (loss) earnings per share:
Basic	108,167			110,394			110,080		110,269
Diluted	108,167			110,698			110,756		110,269



	First Quarter			Second Quarter			Third Quarter (2)			Fourth Quarter
	(In thousands, except per share data)
Fiscal 2018
Revenue	$	121,180		$	137,361		$	152,468		$	166,896
Cost of revenue	65,907			62,250			70,234			71,182
Gross profit	$	55,273		$	75,111		$	82,234		$	95,714
Loss (income) from operations	$	(42,383	)	$	(16,636	)	$	(7,566	)	$	1,177
Net loss	$	(44,904	)	$	(19,922	)	$	(10,158	)	$	(1,826	)
Loss per share (3):
Basic	$	(0.44	)	$	(0.20	)	$	(0.10	)	$	(0.02	)
Diluted	$	(0.44	)	$	(0.20	)	$	(0.10	)	$	(0.02	)
Shares used in computing loss per share:
Basic	101,917			102,160			104,918			106,607
Diluted	101,917			102,160			104,918			106,607

__________________________________


(1)	Includes the results of Anova for the period subsequent to February 28, 2019.


(2)	Includes the results of Edgewater for the period subsequent to August 3, 2018.


(3)	(Loss) earnings per share is calculated independently for each of the quarters presented; accordingly, the sum of the quarterly (loss) earnings per share amounts may not equal the total calculated for the year.

NATURE OF THE BUSINESS (Narrative) (Details) customer in Thousands	Dec. 31, 2019 customer
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of customers	1

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Property, Plant and Equipment [Line Items]
Maximum measurement period from the acquisition date within which company records adjustments to the assets acquired and liabilities assumed		1 year
Goodwill impairment	$ 164,300,000.0	$ 164,300,000	$ 0	$ 0
Advertising expense		$ 500,000	500,000	$ 300,000
Number of reportable operating segments \| segment		1
Operating lease assets	36,654,000	$ 36,654,000	$ 0
Additional lease liabilities	$ 44,921,000	$ 44,921,000
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		5 years
Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		3 years
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		2 years
Minimum \| Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		1 year
Minimum \| Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		5 years
Maximum \| Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		5 years
Maximum \| Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		5 years
Accounting Standards Update 2016-02
Property, Plant and Equipment [Line Items]
Operating lease assets					$ 43,900,000
Additional lease liabilities					$ 47,800,000

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Foreign Currency Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Foreign Currency Balance [Line Items]
Transaction gains (losses)	$ (1,090)	$ (4,611)	$ 783
General and administrative
Schedule of Foreign Currency Balance [Line Items]
Transaction gains (losses)	$ (1,100)	$ (4,600)	$ (700)

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting for Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Tax liability related to undistributed foreign earnings	$ 4,827	$ 4,487

EARNINGS (LOSS) PER SHARE - (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of weighted average shares outstanding from basic to diluted
Weighted average shares outstanding - basic (in shares)	110,269	110,080	110,394	108,167	106,607	104,918	102,160	101,917	109,734	103,916	58,822
Potential dilutive common shares (in shares)									0	0	0
Weighted average shares outstanding - diluted (in shares)	110,269	110,756	110,698	108,167	106,607	104,918	102,160	101,917	109,734	103,916	58,822
Common stock and unvested shares of restricted stock not included because their effect would have been antidilutive (in shares)									4,600	3,100	2,500

BUSINESS ACQUISITONS - PENDING MERGER (Details) - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
	Nov. 14, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired		$ 0	$ 46,389	$ 42,951
Eclipse Communications Ltd.
Business Acquisition [Line Items]
Shares issued to selling shareholders (in shares)	32,500
Payments to Acquire Businesses, Net of Cash Acquired	$ 324,000
Eclipse Communications Ltd.
Business Acquisition [Line Items]
Proceeds from Sale of Real Estate	$ 31,000

CASH EQUIVALENTS AND INVESTMENTS - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments, All Other Investments [Abstract]
Proceeds from sale of available-for-sale securities	$ 12.5	$ 51.6

BUSINESS ACQUISITONS - ANOVA DATA, INC (Details) - USD ($) $ in Thousands, shares in Millions		12 Months Ended
	Jan. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Shares of common stock issued as purchase consideration	$ 15,200
Goodwill		$ 224,896	$ 383,655	$ 335,716
Anova Data, Inc.
Business Acquisition [Line Items]
Shares issued to selling shareholders (in shares)	2.9
Number of additional shares potentially issued (in number of shares)	0.3
Fair value of additional shares potentially issued	$ 1,700
Fair value of total consideration	16,900
Finite-lived intangibles	11,200
Net assets acquired	200
Goodwill		$ 5,500
Weighted average life of identifiable intangible assets (in years)		6 years 3 months
Customer relationships \| Anova Data, Inc.
Business Acquisition [Line Items]
Adjustments to acquired intangibles		$ (2,000)
Finite-lived intangibles	5,200
Developed technology \| Anova Data, Inc.
Business Acquisition [Line Items]
Adjustments to acquired intangibles		$ 2,000
Finite-lived intangibles	$ 6,000

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Activity for Cash Equivalents and Investments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Cash equivalents, amortized cost	$ 310
Cash equivalents, fair value	310
Amortized cost	7,299
Unrealized gains	0
Unrealized losses	(15)
Fair value	7,284
U.S. government agency notes
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Amortized cost	3,998
Unrealized gains	0
Unrealized losses	(9)
Fair value	3,989
Corporate debt securities
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Amortized cost	3,301
Unrealized gains	0
Unrealized losses	(6)
Fair value	$ 3,295

BUSINESS ACQUISITONS - EDGEWATER NETWORKS, INC (Details) - USD ($) $ in Thousands, shares in Millions		12 Months Ended
	Jun. 24, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 08, 2019	Feb. 15, 2019
Business Acquisition [Line Items]
Cash, net of cash acquired		$ 0	$ 46,389	$ 42,951
Deferred purchase consideration		0	30,000
Other income (expense), net		70,444	$ (3,772)	$ 1,274
Edgewater
Business Acquisition [Line Items]
Cash, net of cash acquired	$ 46,400	46,389
Shares issued to selling shareholders (in shares)	4.2
Deferred purchase consideration	$ 30,000				$ 21,900	$ 21,900
Deferred cash payments, within 6 months from closing date	15,000
Deferred cash payments, within 9 to 18 months from closing date	$ 15,000
Other income (expense), net		$ 8,100
Weighted average life of identifiable intangible assets (in years)			8 years 4 months 17 days
Revenue attributable since acquisition			$ 21,500
Net loss attributable to acquisition			$ 4,300

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash Equivalents and Investments by Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value		$ 310
Marketable securities, fair value	$ 0	7,284
Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value		310
Marketable securities, fair value		0
Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value		0
Marketable securities, fair value		7,284
Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value		0
Marketable securities, fair value		0
U.S. government agency notes
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,989
U.S. government agency notes \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
U.S. government agency notes \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,989
U.S. government agency notes \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
Corporate debt securities
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,295
Corporate debt securities \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
Corporate debt securities \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		3,295
Corporate debt securities \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		$ 0

BUSINESS ACQUISITONS - SCHEDULE OF EDGEWATER PURCHASE PRICE ALLOCATION (Details) - USD ($) $ in Thousands			12 Months Ended
	Jun. 24, 2018	Oct. 27, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash consideration:
Net cash consideration			$ 0	$ 46,389	$ 42,951
Fair value of Ribbon stock issued		$ 413,982
Intangible assets:
Goodwill			224,896	$ 383,655	$ 335,716
Edgewater
Cash consideration:
Cash paid to Edgewater Selling Stakeholders			51,162
Less cash acquired			(4,773)
Net cash consideration	$ 46,400		46,389
Unpaid cash consideration			30,000
Fair value of Ribbon stock issued			30,000
Fair value of equity awards assumed (see Note 16)	$ 700		747
Fair value of total consideration			107,136
Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired			16,098
Property and equipment			245
Intangible assets:
Goodwill			48,053
Other noncurrent assets			103
Deferred revenue			(2,749)
Other current liabilities			(9,926)
Deferred revenue, net of current			(669)
Other long-term liabilities			(719)
Assets acquired and liabilities assumed			107,136
Developed technology \| Edgewater
Intangible assets:
Finite-lived intangibles			29,500
Customer relationships \| Edgewater
Intangible assets:
Finite-lived intangibles			26,100
Trade names \| Edgewater
Intangible assets:
Finite-lived intangibles			$ 1,100

BUSINESS ACQUISITONS - GENBAND MERGER (Details) - USD ($)		12 Months Ended
	Oct. 27, 2017	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Business Acquisition [Line Items]
Debt average interest rate				5.96%
Former Sonus Networks Inc. shareholders
Business Acquisition [Line Items]
Percentage of ownership after transaction		50.00%
Former GENBAND Holdings Company and two related holding companies shareholders
Business Acquisition [Line Items]
Sale of stock, percentage of ownership after transaction	50.00%
GENBAND
Business Acquisition [Line Items]
Shares issued as consideration in connection with acquisition (in shares)	50,900,000.0		50,857,708
Repayments of long-term debt	$ 47,973,000
Payment of GENBAND management fee	10,302,000
Promissory note issued to GENBAND equity holders	$ 22,500,000.0
Debt average interest rate	4.67%
Weighted average life of identifiable intangible assets (in years)		8 years 3 months 18 days
GENBAND \| Line of Credit
Business Acquisition [Line Items]
Debt assumed as part of GENBAND merger	$ 17,900,000
GENBAND \| Letter of Credit
Business Acquisition [Line Items]
Debt assumed as part of GENBAND merger	$ 2,900,000

BUSINESS ACQUISITONS - SCHEDULE OF GENBAND PURCHASE PRICE ALLOCATION (Details) - USD ($)		12 Months Ended
	Oct. 27, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash consideration:
Net cash consideration		$ 0	$ 46,389,000	$ 42,951,000
Fair value of Sonus stock issued	$ 413,982,000
Intangible assets:
Goodwill		$ 224,896,000	$ 383,655,000	$ 335,716,000
GENBAND
Cash consideration:
Repayment of GENBAND long-term debt and accrued interest, related party	47,973,000
Payment of GENBAND management fees due to majority shareholder	10,302,000
Less cash acquired	(15,324,000)
Net cash consideration	42,951,000
Promissory note issued to GENBAND equity holders	22,500,000.0
Fair value of total consideration	479,433,000
Fair value of assets acquired and liabilities assumed:
Current assets, net of cash acquired	99,126,000
Property and equipment	16,770,000
Intangible assets:
Goodwill	285,825,000
Other noncurrent assets	6,732,000
Revolving credit facility	(17,930,000)
Deferred revenue	(32,390,000)
Other current liabilities	(80,023,000)
Deferred revenue, net of current	(6,804,000)
Other long-term liabilities	(28,673,000)
Assets acquired and liabilities assumed	479,433,000
GENBAND \| In-process research and development
Intangible assets:
Finite-lived intangibles	5,600,000
GENBAND \| Developed technology
Intangible assets:
Finite-lived intangibles	129,000,000
GENBAND \| Customer relationships
Intangible assets:
Finite-lived intangibles	101,300,000
GENBAND \| Trade names
Intangible assets:
Finite-lived intangibles	$ 900,000

BUSINESS ACQUISITONS - GENBAND PRO FORMA RESULTS (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares
Business Combinations [Abstract]
Revenue	$ 615,286
Net loss	$ (69,741)
Loss per share (in usd per share) \| $ / shares	$ (0.69)

BUSINESS ACQUISITONS - SCHEDULE OF ACQUISITION- AND INTEGRATION-RELATED COSTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Combinations [Abstract]
Professional and services fees (acquisition-related)	$ 8,657	$ 7,627	$ 11,916
Management bonuses (acquisition-related)	0	1,972	931
Integration-related expenses	4,296	7,352	1,916
Acquisition- and integration-related	$ 12,953	$ 16,951	$ 14,763

ACCOUNTS RECEIVABLE, NET - Schedule of Accounts Receivabale, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Accounts receivable	$ 193,619	$ 188,522
Allowance for doubtful accounts	(913)	(669)	$ (73)	$ (10)
Accounts receivable, net	$ 192,706	$ 187,853

ACCOUNTS RECEIVABLE, NET - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of year	$ 669	$ 73	$ 10
Charges to expense	738	351	154
Charges (credits) to other accounts (deferred revenue)	68	620	(56)
Write-offs	(562)	(375)	(35)
Balance at end of year	$ 913	$ 669	$ 73

INVENTORY - (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
On-hand final assemblies and finished goods inventories	$ 13,283	$ 19,879
Deferred cost of goods sold	2,441	3,798
Gross inventory	15,724	23,677
Less noncurrent portion (included in Other assets)	(924)	(1,075)
Current portion	$ 14,800	$ 22,602

PROPERTY AND EQUIPMENT - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 123,840
Property and equipment, gross	$ 135,934
Less accumulated depreciation and amortization	(106,958)
Less accumulated depreciation and amortization		(96,798)
Property and equipment, net	28,976
Property and equipment, net	$ 28,976	27,042
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 82,737	76,423
Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Software
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Property and equipment, gross	$ 27,939	24,707
Software \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Software \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,283	1,490
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Property and equipment, gross		$ 21,220
Property and equipment, gross	$ 23,975
Leasehold improvements \| Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	1 year
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

PROPERTY AND EQUIPMENT - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization of property and equipment	$ 11,949	$ 11,200	$ 8,486

PROPERTY AND EQUIPMENT - Property and Equipment Under Capital Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Cost	$ 4,401
Cost		$ 2,979
Less accumulated depreciation	(1,981)
Less accumulated depreciation		(875)
Property and equipment under finance leases, net	$ 2,420	2,104
Property and equipment under finance leases, net		$ 2,104

PROPERTY AND EQUIPMENT - Property and Equipment by Geographic Area (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 28,976
Property and equipment, net	28,976	$ 27,042
United States
Property, Plant and Equipment [Line Items]
Property and equipment, net	17,584
Property and equipment, net		17,862
Canada
Property, Plant and Equipment [Line Items]
Property and equipment, net	4,768
Property and equipment, net		4,076
Asia/Pacific
Property, Plant and Equipment [Line Items]
Property and equipment, net	5,146
Property and equipment, net		3,841
Europe
Property, Plant and Equipment [Line Items]
Property and equipment, net	1,224
Property and equipment, net		1,100
Other
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 254
Property and equipment, net		$ 163

INTANGIBLE ASSETS AND GOODWILL (Details) - USD ($)	3 Months Ended	12 Months Ended
INTANGIBLE ASSETS AND GOODWILL (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Intangible Assets And Goodwill
Weighted average amortization period (years)		7 years 9 months 25 days	7 years 10 months 17 days
Cost	$ 349,350,000				$ 338,150,000
Accumulated amortization	135,984,000				86,759,000
Net carrying value	213,366,000				251,391,000
Amortization and impairment		$ 49,225,000	$ 49,723,000	$ 22,583,000
Estimated future amortization expense for intangible assets
2020	48,952,000
2021	43,495,000
2022	35,092,000
2023	27,271,000
2024	20,201,000
Thereafter	38,355,000
Total	213,366,000
Goodwill [Roll Forward]
Goodwill	392,302,000			338,822,000	386,761,000
Accumulated impairment losses	(167,406,000)			(3,106,000)	(3,106,000)
Goodwill at the beginning of the period		383,655,000	335,716,000
Impairment of goodwill	(164,300,000.0)	(164,300,000)	0	0
Goodwill at the end of the period	224,896,000	$ 383,655,000	$ 335,716,000	335,716,000	383,655,000
In-process research and development
Intangible Assets And Goodwill
Cost	5,600,000				5,600,000
Accumulated amortization	0				0
Net carrying value	5,600,000				5,600,000
Developed technology
Intangible Assets And Goodwill
Weighted average amortization period (years)		6 years 9 months 15 days	6 years 10 months 28 days
Cost	188,880,000				182,880,000
Accumulated amortization	100,760,000				63,187,000
Net carrying value	88,120,000				119,693,000
Amortization and impairment		$ 37,573,000	$ 38,976,000	18,358,000
Estimated future amortization expense for intangible assets
Impairment of intangible assets and goodwill				5,500,000
Customer relationships
Intangible Assets And Goodwill
Weighted average amortization period (years)		9 years 5 months 14 days	9 years 5 months 10 days
Cost	152,140,000				146,940,000
Accumulated amortization	33,350,000				22,218,000
Net carrying value	118,790,000				124,722,000
Amortization and impairment		$ 11,132,000	$ 10,203,000	4,145,000
Trade names
Intangible Assets And Goodwill
Weighted average amortization period (years)		5 years 2 months 12 days	5 years 2 months 12 days
Cost	2,000,000				2,000,000
Accumulated amortization	1,144,000				624,000
Net carrying value	856,000				1,376,000
Amortization and impairment		$ 520,000	$ 544,000	$ 80,000
Internal use software
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years	3 years
Cost	730,000				730,000
Accumulated amortization	730,000				730,000
Net carrying value	0				$ 0
Anova Data, Inc.
Goodwill [Roll Forward]
Acquisition		$ 5,541,000	$ 0
Goodwill at the end of the period	5,500,000
Edgewater
Goodwill [Roll Forward]
Acquisition		0	48,053,000
Goodwill at the end of the period	$ 48,053,000
Taqua, LLC
Goodwill [Roll Forward]
Write-off of goodwill attributable to dissolved subsidiary		$ 0	$ (114,000)

ACCRUED EXPENSES AND OTHER - (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Employee compensation and related costs	$ 27,166	$ 42,852
Professional fees	13,331	7,994
Deferred purchase consideration - business acquisitions	1,700	15,000
Other	14,503	18,417
Total	$ 56,700	$ 84,263

RESTRUCTURING AND FACILITIES CONSOLIDATION INTIATIVES - Narrative (Details) $ in Thousands	1 Months Ended	8 Months Ended	12 Months Ended
	Jul. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2019 USD ($) employee	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($) employee	Oct. 27, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 16,399	$ 17,015	$ 9,436
Accelerated amortization of lease assets due to cease-use			3,692
Long-term portions of accrued restructuring			900	500
2019 Restructuring Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring and related expense, including accelerated depreciation			11,200
Restructuring expense			7,475
Accelerated amortization of lease assets due to cease-use			3,700
Accelerated rent amortization			3,700
2019 Restructuring Initiative \| Severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 6,103
Number of positions eliminated \| employee			120
2019 Restructuring Initiative \| Variable And other facilities costs
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 1,400
2019 Restructuring Initiative \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense			1,372
Merger Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 5,232	$ 16,134
Number of positions eliminated \| employee			40	275	120
Merger Restructuring \| Severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 5,076	$ 14,735	$ 8,500
Merger Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense			156	1,399
GENBAND Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 600
Restructuring liability assumed						$ 4,100
GENBAND Restructuring \| Severance
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed						3,700
GENBAND Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring liability assumed						$ 400
2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense	$ 2,000				500
2016 Restructuring \| Severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense	1,900				400
2016 Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense	$ 100				100
Taqua Restructuring Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring expense		$ 1,800			700
Taqua Restructuring Initiative \| Severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense		1,200			200
Taqua Restructuring Initiative \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense		$ 600			500
Restructuring estimate \| GENBAND Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense				$ 900	$ 300

RESTRUCTURING AND FACILITIES CONSOLIDATION INTIATIVES - Components of Restructuring Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Severance and related costs	$ 11,179	$ 15,217	$ 8,925
Variable and other facilities-related costs	1,528	1,798	511
Accelerated amortization of lease assets due to cease-use	3,692
Restructuring and related	$ 16,399	$ 17,015	$ 9,436

RESTRUCTURING AND FACILITIES CONSOLIDATION INTIATIVES - Restructuring Initiatives (Details) - USD ($) $ in Thousands	1 Months Ended	8 Months Ended	12 Months Ended
	Jul. 31, 2016	Jun. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			$ 979
Initiatives charged to expense			16,399	$ 17,015	$ 9,436
Cash payments				(1,000)
Balance at the end of the period			3,510	979
2019 Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0
Initiatives charged to expense			7,475
Cash payments			(4,374)
Balance at the end of the period			3,101	0
Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			2,681	7,595
Initiatives charged to expense			5,232	16,134
Adjustments for changes in estimate				(5)
Cash payments			(6,733)	(21,043)
Balance at the end of the period			409	2,681	7,595
GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			117	2,121
Initiatives charged to expense			600
Adjustments for changes in estimate				886
Cash payments				(2,890)
Balance at the end of the period				117	2,121
2016 Restructuring
Restructuring Reserve [Roll Forward]
Initiatives charged to expense	$ 2,000				500
Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Initiatives charged to expense		$ 1,800			700
Severance \| 2019 Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0
Initiatives charged to expense			6,103
Cash payments			(3,993)
Balance at the end of the period			2,110	0
Severance \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			1,910	7,595
Initiatives charged to expense			5,076	14,735	8,500
Adjustments for changes in estimate				(5)
Cash payments			(6,577)	(20,415)
Balance at the end of the period			409	1,910	7,595
Severance \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0	1,916
Adjustments for changes in estimate				487
Cash payments				(2,403)
Balance at the end of the period				0	1,916
Severance \| 2016 Restructuring
Restructuring Reserve [Roll Forward]
Initiatives charged to expense	1,900				400
Severance \| Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Initiatives charged to expense		1,200			200
Facilities
Restructuring Reserve [Roll Forward]
Balance at the end of the period			900
Facilities \| 2019 Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0
Initiatives charged to expense			1,372
Cash payments			(381)
Balance at the end of the period			991	0
Facilities \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			771	0
Initiatives charged to expense			156	1,399
Adjustments for changes in estimate				0
Cash payments			(156)	(628)
Balance at the end of the period			0	771	0
Facilities \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			117	205
Adjustments for changes in estimate				399
Cash payments				(487)
Balance at the end of the period			0	117	205
Facilities \| 2016 Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			58	95
Initiatives charged to expense	$ 100				100
Cash payments			(58)	(37)
Balance at the end of the period			0	58	95
Facilities \| Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			33	365
Initiatives charged to expense		$ 600			500
Cash payments				(332)
Balance at the end of the period			0	$ 33	$ 365
Accounting Standards Update 2016-02 \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Adjustment for the impact of ASC 842 adoption			(771)
Accounting Standards Update 2016-02 \| Severance \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Adjustment for the impact of ASC 842 adoption			0
Accounting Standards Update 2016-02 \| Facilities \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Adjustment for the impact of ASC 842 adoption			(771)
Accounting Standards Update 2016-02 \| Facilities \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Adjustment for the impact of ASC 842 adoption			(117)
Accounting Standards Update 2016-02 \| Facilities \| Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Adjustment for the impact of ASC 842 adoption			$ (33)

DEBT - Assumed Senior Secured Credit Agreement (Details) - USD ($)	Dec. 31, 2019	Apr. 29, 2019	Dec. 31, 2018	Dec. 21, 2017	Oct. 27, 2017
Debt Instrument [Line Items]
Debt average interest rate			5.96%
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Debt average interest rate	3.30%
Line of credit potential increases		$ 100,000,000		$ 100,000,000
Silicon Valley Bank \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Current borrowing capacity for line of credit	$ 50,000,000
Potential further increases	75,000,000
Line of credit potential increases	$ 125,000,000
GENBAND
Debt Instrument [Line Items]
Debt average interest rate					4.67%
Line of Credit \| GENBAND
Debt Instrument [Line Items]
Debt assumed as part of GENBAND merger					$ 17,900,000
Letter of Credit \| GENBAND
Debt Instrument [Line Items]
Debt assumed as part of GENBAND merger					$ 2,900,000

DEBT - Senior Secured Credit Facility (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Apr. 29, 2019	Dec. 31, 2018	Dec. 21, 2017
Debt Instrument [Line Items]
Outstanding debt			$ 55,000,000
Debt interest rate			5.96%
Line of Credit
Debt Instrument [Line Items]
Commitment potential aggregate increase amount				$ 50,000,000
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender		$ 100,000,000		100,000,000
Fronting fee percentage	0.125%
Default rate percentage	2.00%
Debt interest rate	3.30%
Revolving Credit Facility \| Line of Credit \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage	0.25%
Letter of credit, fee multiplier	1.50%
Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Commitment fee percentage	0.40%
Letter of credit, fee multiplier	2.00%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility \| Line of Credit \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender				15,000,000
Outstanding debt			$ 2,700,000
Debt interest rate	1.50%		1.75%
Swingline loan \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender				$ 15,000,000

DEBT - New Senior Secured Credit Facility (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Apr. 29, 2019	Dec. 31, 2018	Dec. 21, 2017
Debt Instrument [Line Items]
Outstanding debt			$ 55,000,000
Debt interest rate			5.96%
Line of Credit
Debt Instrument [Line Items]
Commitment potential aggregate increase amount				$ 50,000,000
Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender				15,000,000
Outstanding debt			$ 2,700,000
Debt interest rate	1.50%		1.75%
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender		$ 100,000,000		$ 100,000,000
Debt interest rate	3.30%
Default rate percentage	2.00%
Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility \| Line of Credit \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
New Credit Facility
Debt Instrument [Line Items]
Outstanding debt	$ 48,800,000
New Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate principal amount		50,000,000
New Credit Facility \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Outstanding debt	8,000,000
2019 Credit Facility
Debt Instrument [Line Items]
Commitment potential aggregate increase amount		$ 75,000,000
2019 Credit Facility \| Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Outstanding debt	$ 5,400,000
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Default rate percentage	2.00%
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit \| London Interbank Offered Rate (LIBOR) \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
2019 Credit Facility \| Revolving Credit Facility \| Line of Credit \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%

DEBT - Promissory Note (Details) - USD ($)	Apr. 29, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 27, 2017
GENBAND
Debt Instrument [Line Items]
Promissory note issued to GENBAND equity holders				$ 22,500,000.0
GENBAND \| Minimum
Debt Instrument [Line Items]
Promissory note interest rate		7.50%
GENBAND \| Maximum
Debt Instrument [Line Items]
Promissory note interest rate		10.00%
Promissory Note
Debt Instrument [Line Items]
Promissory note issued to GENBAND equity holders			$ 22,500,000
Promissory note balance			24,100,000
Interest converted to principal			$ 1,600,000
Notes Payable, Other Payables \| Promissory Note
Debt Instrument [Line Items]
Repayments of outstanding amounts	$ 24,700,000

LONG-TERM LIABILITIES - Schedule of Long-Term Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Liabilities [Abstract]
Finance lease obligations	$ 3,149
Finance lease obligations		$ 2,363
Deferred rent	0	3,039
Restructuring	3,510	979
Pension obligations	9,954	7,006
Taxes payable	1,991	1,818
Deferred purchase consideration	0	30,000
Other	1,683	2,425
Long-term liabilities	20,287	47,630
Current portion	(3,698)	(16,833)
Long-term liabilities, net of current portion	$ 16,589	$ 30,797

REVENUE RECOGNITION (Details) - USD ($)	3 Months Ended								12 Months Ended
REVENUE RECOGNITION (Details) - USD ($)	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue recognized that was previously recorded as deferred revenue									$ 94,000,000	$ 84,000,000
Capitalized contract cost, amortization period	5 years								5 years		5 years
Deferred sales commissions accrued	$ 3,600,000				$ 2,700,000				$ 3,600,000	2,700,000
Prior period adjustment related to the adoption of Accounting Standards Codification 606, Revenue from Contracts with Customers, tax					0					0
Less revenue recognized	$ (161,109,000)	$ (137,653,000)	$ (145,421,000)	$ (118,928,000)	$ (166,896,000)	$ (152,468,000)	$ (137,361,000)	$ (121,180,000)
Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Adoption of Accounting Standards Codification 606, Revenue from Contracts with Customers											$ 12,244,000
Accumulated deficit \| Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Adoption of Accounting Standards Codification 606, Revenue from Contracts with Customers											12,244,000
Prior period adjustment related to the adoption of Accounting Standards Codification 606, Revenue from Contracts with Customers, tax											$ 0
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Less revenue recognized										$ 16,000,000

REVENUE RECOGNITION - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 563,111	$ 577,905	$ 329,942
United States
Disaggregation of Revenue [Line Items]
Total revenue	341,293	337,624	219,057
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	97,727	96,483	44,565
Japan
Disaggregation of Revenue [Line Items]
Total revenue	30,599	39,411	24,389
Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	38,537	47,254	22,546
Other
Disaggregation of Revenue [Line Items]
Total revenue	54,955	57,133	19,385
Product
Disaggregation of Revenue [Line Items]
Total revenue	262,030	279,014	181,119
Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	262,030	279,014	181,119
Product revenue \| United States
Disaggregation of Revenue [Line Items]
Total revenue	170,937	169,510	121,121
Product revenue \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	42,262	37,833	23,352
Product revenue \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	13,065	23,108	10,252
Product revenue \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	17,552	30,575	14,693
Product revenue \| Other
Disaggregation of Revenue [Line Items]
Total revenue	18,214	17,988	11,701
Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	234,228	233,966	114,735
Service revenue (maintenance) \| United States
Disaggregation of Revenue [Line Items]
Total revenue	133,271	132,282	75,040
Service revenue (maintenance) \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	43,186	46,856	17,471
Service revenue (maintenance) \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	11,692	11,234	10,282
Service revenue (maintenance) \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	16,106	12,321	5,901
Service revenue (maintenance) \| Other
Disaggregation of Revenue [Line Items]
Total revenue	29,973	31,273	6,041
Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	66,853	64,925	34,088
Service revenue (professional services) \| United States
Disaggregation of Revenue [Line Items]
Total revenue	37,085	35,832	22,896
Service revenue (professional services) \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	12,279	11,794	3,742
Service revenue (professional services) \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	5,842	5,069	3,855
Service revenue (professional services) \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	4,879	4,358	1,952
Service revenue (professional services) \| Other
Disaggregation of Revenue [Line Items]
Total revenue	6,768	7,872	1,643
Indirect sales through channel program \| Product
Disaggregation of Revenue [Line Items]
Total revenue	94,639	69,232	43,138
Direct sales \| Product
Disaggregation of Revenue [Line Items]
Total revenue	167,391	209,782	137,981
Sales to enterprise customers \| Product
Disaggregation of Revenue [Line Items]
Total revenue	70,548	57,534	35,592
Sales to service provider customers \| Product
Disaggregation of Revenue [Line Items]
Total revenue	$ 191,482	$ 221,480	$ 145,527

REVENUE RECOGNITION - Schedule of Customer Assets & Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable
Beginning balance	$ 174,310	$ 149,122
Increase (decrease), net	(5,808)	25,188
Ending balance	168,502	174,310
Unbilled accounts receivable
Beginning balance	13,543	16,034
Increase (decrease), net	10,661	(2,491)
Ending balance	24,204	13,543
Deferred revenue (current)
Beginning balance	105,087	100,571
Increase (decrease), net	(4,681)	4,516
Ending balance	100,406	105,087
Deferred revenue (long-term)
Beginning balance	17,572	14,184
Increase (decrease), net	2,910	3,388
Ending balance	$ 20,482	$ 17,572

REVENUE RECOGNITION - Performance Obligations (Details) $ in Millions	Dec. 31, 2019 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, remaining performance obligation, period	$ 13
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year
Revenue, remaining performance obligation, period	$ 4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period
Revenue, remaining performance obligation, period	$ 3

COMMON STOCK REPURCHASES (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 4,536,000	$ 0	$ 0
Repurchase Program
Class of Stock [Line Items]
Authorized amount				$ 75,000,000
Payments for repurchase of common stock	$ 4,500,000
Shares repurchased and retired (in shares)	1.0
Remaining authorized repurchase amount	$ 70,500,000

STOCK-BASED COMPENSATION PLANS - Amended and Restate Stock Incentive Plan (Details) - shares	Dec. 31, 2019	Jun. 05, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	8,200,426
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)		7,000,000
Shares available for future issuance (in shares)	7,051,559
Amended and Restated Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)		5,100,000

STOCK-BASED COMPENSATION PLANS - Stock Options Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 24, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from the exercise of stock options		$ 235	$ 73	$ 617
2012 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance under plan		0
Award expiration period (in years)		10 years
2008 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance under plan		0
Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)		10 years
Edgewater plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)		10 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share)				$ 3.05
Intrinsic value of options exercised		$ 500	39	$ 200
Proceeds from the exercise of stock options		200	$ 100	$ 600
Edgewater
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion factor	17.00%
Acquisition consideration (in dollars per share)	$ 1.20
Additional purchase consideration	$ 700	$ 747
Fair value of Ribbon Replacement Options	$ 1,000
Fair value of Ribbon Replacement Options, weighted average period (in years)	2 years
Minimum \| 2008 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)		7 years
Maximum \| 2008 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)		10 years

STOCK-BASED COMPENSATION PLANS - Treatment of Equity Awards in Connection with the Merger (Details) - shares shares in Thousands	Oct. 27, 2017	Oct. 24, 2017	Oct. 20, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)			300
Canceled vested unexervised stock options (in shares)		4,500
RSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)	1,100
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)	36
PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting (in shares)		98

STOCK-BASED COMPENSATION PLANS - Executive Equity Arrangements (Details) $ / shares in Units, $ in Thousands			1 Months Ended				2 Months Ended	12 Months Ended
	Mar. 15, 2019 shares	Mar. 31, 2017 executive shares	Apr. 30, 2019 shares	Feb. 28, 2019 executive shares	May 31, 2018 shares	Mar. 31, 2018 executive shares	Apr. 30, 2019 shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) participant	Dec. 31, 2017 USD ($)	Mar. 08, 2019 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Price \| $ / shares											$ 4.97
Stock-based compensation \| $								$ 12,601	$ 11,072	$ 25,657
Vested (in shares)			292,500
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)							872,073
PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)					195,000			872,073
Stock-based compensation \| $									$ 300
Percent of performance metrics achieved					150.00%
Percent of performance metrics achieved, level one				106.49%
Percent of performance metrics achieved, level two				150.00%
Vested (in shares)				250,075				9,466
Stock Bonus Election Program \| 2018 Bonus Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Uplift percentage									20.00%
Stock Bonus Election Program \| Share-based Payment Arrangement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Participants \| participant									23
Granted (in shares)	198,949
Stock Bonus Election Program \| Uplift Shares and 2018 Bonus Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $									$ 1,100
Performance PSUs \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)							523,244
Percent of performance metrics achieved								200.00%
Market PSUs \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)							348,829
Percent of performance metrics achieved								200.00%
2017 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		165,000
Percent of performance metrics achieved				61.40%		130.00%
Vested (in shares)				9,464		33,584
Executive granted shares \| executive		5
Number of remaining executives holding grants \| executive				3		3
March 31, 2017 \| 2017 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved						100.00%
Vested (in shares)						25,834
March 31, 2018 \| 2017 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)						7,750
Percent achievement over target						30.00%
Minimum \| Stock Bonus Election Program \| 2018 Bonus Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of bonus available for program									10.00%
Maximum \| Stock Bonus Election Program \| 2018 Bonus Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of bonus available for program									50.00%
Executive Officer \| 2018 Performance Share Units \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved			150.00%
Executive Officer \| March 16, 2018 \| 2018 Performance Share Units \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)				5,950

STOCK-BASED COMPENSATION PLANS - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2017
Range of assumptions used in estimating fair value of options
Expected dividends	0.00%
Expected life (years)		5 years
Stock options
Number of Shares
Outstanding at the beginning of the period (in shares)	582,061
Granted (in shares)	0
Exercised (in shares)	(127,334)
Forfeited (in shares)	(38,666)
Expired (in shares)	(118,937)
Outstanding at the end of the period (in shares)	297,124
Vested or expected to vest at the end of the period (in shares)	293,782
Exercisable at the end of the period (in shares)	260,152
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 9.01
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	1.85
Forfeited (in dollars per share)	2.55
Expired (in dollars per share)	12.43
Outstanding at the end of the period (in dollars per share)	11.55
Vested or expected to vest at the end of the period (in dollars per share)	11.65
Exercisable at the end of the period (in dollars per share)	$ 12.89
Weighted Average Remaining Contractual Term (years)
Outstanding at the end of the period	4 years 11 months 13 days
Vested or expected to vest at the end of the period	4 years 11 months 2 days
Exercisable at the end of the period	4 years 7 months 1 day
Aggregate intrinsic value (in dollars)
Outstanding at the end of the period (in dollars)	$ 122
Vested or expected to vest at the end of the period (in dollars)	119
Exercisable at the end of the period (in dollars)	$ 85
Range of assumptions used in estimating fair value of options
Risk-free interest rates, minimum		1.22%
Risk-free interest rates, maximum		1.95%
Expected dividends		0.00%
Weighted average volatility		51.10%

STOCK-BASED COMPENSATION PLANS - RSA and RSU Activity (Details) - $ / shares	1 Months Ended	12 Months Ended
	Apr. 30, 2019	Dec. 31, 2019
Change in unvested restricted stock awards
Vested (in shares)	(292,500)
RSAs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)		1,508,011
Granted (in shares)		0
Vested (in shares)		(967,291)
Forfeited (in shares)		(52,744)
Unvested balance at the end of the period (in shares)		487,976
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share)		$ 6.90
Granted (in dollars per share)		0.00
Vested (in dollars per share)		6.90
Forfeited (in dollars per share)		7.04
Unvested balance at end of the period (in dollars per share)		$ 6.87
RSUs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)		636,300
Granted (in shares)		2,828,832
Vested (in shares)		(537,416)
Forfeited (in shares)		(137,656)
Unvested balance at the end of the period (in shares)		2,790,060
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share)		$ 6.52
Granted (in dollars per share)		4.98
Vested (in dollars per share)		6.04
Forfeited (in dollars per share)		5.35
Unvested balance at end of the period (in dollars per share)		$ 5.11

STOCK-BASED COMPENSATION PLANS - RSA and RSU Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested	$ 7,422	$ 8,312	$ 20,515
RSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested	$ 9,900	$ 9,700	$ 19,100

STOCK-BASED COMPENSATION PLANS - Performance-Based Stock Units Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Apr. 30, 2019	Feb. 28, 2019	May 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in unvested restricted stock awards
Vested (in shares)	(292,500)
Weighted average grant-date fair value
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested				$ 7,422	$ 8,312	$ 20,515
PSUs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)				210,416
Granted (in shares)			195,000	872,073
Vested (in shares)		(250,075)		(9,466)
Forfeited (in shares)				(5,950)
Unvested balance at the end of the period (in shares)				1,067,073	210,416
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share)				$ 5.77
Granted (in dollars per share)				6.03
Vested (in dollars per share)				8.55
Forfeited (in dollars per share)				8.55
Unvested balance at end of the period (in dollars per share)				$ 5.94	$ 5.77
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested				$ 100	$ 600	$ 1,400

STOCK-BASED COMPENSATION PLANS - ESPP (Details)	12 Months Ended
STOCK-BASED COMPENSATION PLANS - ESPP (Details)	Dec. 31, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	8,200,426
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock (as a percentage)	85.00%
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum shares per employee (in shares)	500
Number of shares authorized (in shares)	5,000,000
Shares available for future issuance (in shares)	1,148,867

STOCK-BASED COMPENSATION PLANS - Schedule of Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 12,601	$ 11,072	$ 25,657
Product cost of revenue
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	76	114	514
Service cost of revenue
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	478	345	1,448
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	1,898	1,797	7,337
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	3,028	2,935	4,885
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 7,121	$ 5,881	$ 11,473

STOCK-BASED COMPENSATION PLANS - Stock-Based Compensation (Details) - USD ($)		12 Months Ended
	Dec. 13, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit for employee stock-based compensation		$ 0	$ 0	$ 0
Stock-based compensation		12,601,000	11,072,000	25,657,000
Unrecognized stock-based compensation expense		$ 11,300,000
Weighted average period (in years)		2 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 1,600,000
GENBAND \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 1,600,000		8,600,000
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 3,028,000	$ 2,935,000	$ 4,885,000

STOCK-BASED COMPENSATION PLANS - Common Stock Reserved (Details)	Dec. 31, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	8,200,426
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	7,051,559
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	1,148,867

LEASES - Narrative (Details)	12 Months Ended
LEASES - Narrative (Details)	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee, Lease, Description [Line Items]
Additional amortization expense	$ 3,692,000
Restructuring	$ 3,510,000	$ 979,000
Square feet under lease \| ft²	107,800
Rent expense		11,900,000	$ 5,900,000
Interest expense for finance leases		52,000	14,000
Amortization expense for finance leases		600,000	$ 100,000
Facilities
Lessee, Lease, Description [Line Items]
Restructuring	$ 900,000
2019 Restructuring Initiative
Lessee, Lease, Description [Line Items]
Additional amortization expense	3,700,000
Restructuring	3,101,000	0
2019 Restructuring Initiative \| Facilities
Lessee, Lease, Description [Line Items]
Restructuring	$ 991,000	$ 0

LEASES - Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Operating lease assets	$ 36,654	$ 0
Finance lease assets	2,420	2,104
Total leased assets	39,074	2,104
Current
Operating	7,719	0
Finance	1,005	1,039
Noncurrent
Operating	37,202	0
Finance	2,144	1,324
Total lease liabilities	48,070	2,363
Finance lease assets, net of accumulated depreciation	$ 2,000	$ 900

LEASES - Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease cost	$ 13,865
Finance lease cost
Amortization of leased assets	1,106
Interest on lease liabilities	265
Short-term lease cost	19,460
Variable lease costs (costs excluded from minimum fixed lease payments)	3,264
Sublease income	(374)	$ (125)
Net lease cost	37,586
Accelerated rent amortization	3,692
Variable lease, payment	$ 900

LEASES - Other Information (Details) $ in Thousands	12 Months Ended
LEASES - Other Information (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 10,559
Operating cash flows from finance leases	265
Financing cash flows from finance leases	$ 913
Weighted average remaining lease term (years)
Operating leases (in years)	6 years 8 months 23 days
Finance leases (in years)	2 years 4 months 7 days
Weighted average discount rate
Operating leases (as a percentage)	6.50%
Finance leases (as a percentage)	7.54%

LEASES - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Leases, After Adoption of 842:
2020	$ 10,290
2021	9,468
2022	7,665
2023	7,067
2024	5,303
2025 and beyond	15,738
Total lease payments	55,531
Less: interest	(10,610)
Present value of lease liabilities	44,921
Finance Leases, After Adoption of 842:
2020	1,644
2021	1,159
2022	581
2023	0
2024	0
2025 and beyond	0
Total lease payments	3,384
Less: interest	(235)
Present value of lease liabilities	3,149
Operating Leases, Before Adoption of 842
2019		$ 10,705
2020		8,384
2021		7,455
2022		5,691
2023		5,430
2024 and beyond		19,818
Total lease payments		57,483
Finance Leases, Before Adoption of 842
2019		1,386
2020		1,010
2021		288
2022		0
2023		0
2024 and beyond		0
Total lease payments		2,684
Less: interest		(321)
Present value of lease liabilities		2,363
Cash payments		$ 1,000
Percent of restructuring charges due in less than one year		50.00%
Restructuring payments, current, term (in years)		1 year
Sublease income	$ 374	$ 125
Minimum
Finance Leases, Before Adoption of 842
Restructuring payments, noncurrent, term (in years)		1 year
Maximum
Finance Leases, Before Adoption of 842
Restructuring payments, noncurrent, term (in years)		3 years

EMPLOYEE DEFINED CONTRIBUTION PLANS - (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Employer matching percent of employees' contribution	50.00%
Employer matching contribution percentage of employees' gross pay	4.00%
Maximum employer match percentage per employee	2.00%
Defined contribution expense		$ 4.0	$ 3.2	$ 1.4

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Narrative (Details) - USD ($)	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Curtailment gain			$ 500,000
Benefits paid	$ 3,000	$ 683,000	273,000
One-time lump sum payments to participants		$ 660,000	250,000
Net gains and losses amortization threshold		10.00%
Expected amortization of gain (loss) in the next fiscal year		$ 176,000
Fair value of combined plan assets	3,893,000	1,830,000	3,842,000
Participant contributions	$ 5,000	24,000	5,000
Fixed contributions per employee, percent	5.00%
Employer pension plan contributions	$ 22,000	139,000	$ 292,000
Expected future employer contributions to pension plans in 2020		$ 200,000

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Changes in Benefit Obligations and Fair Value of Plan (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of year		$ 10,848	$ 11,484
Service cost	$ 68	335	449
Interest cost	25	140	150
Participant contributions		24	5
Benefits and expenses paid		(44)	(23)
Net actuarial loss (gain) on obligation		1,059	(414)
Curtailment		82	(553)
Settlement		(660)	(250)
Projected benefit obligation, end of year	11,484	11,784	10,848
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		3,842	3,893
Actual return on plan assets		(1,471)	(53)
Employer contributions	22	139	292
Participant contributions	5	24	5
Administrative expenses	(4)	(21)	(22)
Benefits paid	(3)	(683)	(273)
Fair value of plan assets, end of year	$ 3,893	1,830	3,842
Funded status at end of year		(9,954)	(7,006)
Net actuarial loss		2,743	222
Liability, Defined Benefit Plan [Abstract]
Accrued expenses and other (current pension liability)		(74)	(75)
Other long-term liabilities (non-current pension liability)		(9,880)	(6,931)
Net amount recognized		$ (9,954)	$ (7,006)

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Aggregate projected benefit obligation	$ 11,784	$ 10,848
Aggregate accumulated benefit obligation	7,759	7,152
Aggregate fair value of plan assets	$ 1,830	$ 3,842	$ 3,893

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Service cost	$ 68	$ 335	$ 449
Interest cost	25	140	150
Expected return on plan assets	(8)	(14)	(45)
Plan asset expenses	4	21	22
Curtailment charge (credit)	0	13	(510)
Settlement charge	0	115	3
Net periodic benefit costs	$ 89	$ 610	$ 69

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Expected Future Benefit Payments (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Retirement Benefits [Abstract]
2020	$ 74
2021	94
2022	45
2023	223
2024	57
2025 to 2029	1,661
Expected future benefit payments	$ 2,154

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Change Recognized in Other Comprehesinve Loss (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Net loss (gain)	$ 578	$ 2,526	$ (356)

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Assumptions for Benefit Obligation (Details)	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Discount rate	0.68%	1.30%
Rate of compensation increase	2.88%	2.83%

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Assumption for Net Periodic Benefit Cost (Details)	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Discount rate	1.49%	1.30%	1.50%
Expected long-term return on plan assets	1.23%	1.12%	1.34%
Rate of compensation increase	3.38%	2.83%	3.38%

INCOME TAXES - Schedule of Income (Loss) Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss before income taxes:
United States	$ (132,887)	$ (52,569)	$ (55,932)
Foreign	9,994	(20,841)	2,240
Loss before income taxes	$ (122,893)	$ (73,410)	$ (53,692)

INCOME TAXES - Schedule of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ 11	$ 561	$ (200)
State	128	128	115
Foreign	1,744	2,198	1,960
Total current	1,883	2,887	1,875
Deferred:
Federal	9,790	(8,481)	49,570
State	1,630	(1,414)	(4,833)
Foreign	383	(1,477)	(816)
Change in valuation allowance	(6,504)	11,885	(64,236)
Total deferred	5,299	513	(20,315)
Total	$ 7,182	$ 3,400	$ (18,440)

INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. statutory income tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal benefit	(0.20%)	(0.10%)	1.20%
Foreign income taxes	(1.00%)	(5.70%)	(0.50%)
Acquisition costs	(0.50%)	(0.30%)	(6.00%)
Foreign deemed dividends	(0.40%)	(3.40%)	(3.80%)
Stock-based compensation	(0.70%)	(0.30%)	26.80%
Tax credits	2.80%	0.60%	(33.30%)
Uncertain tax positions	(0.20%)	1.30%	(1.20%)
NOL and credit limitations	0.00%	0.00%	(18.90%)
Valuation allowance	(0.70%)	(16.10%)	29.00%
Goodwill amortization	0.40%	0.30%	(1.70%)
Meals and entertainment	(0.30%)	(0.40%)	(0.50%)
Tax reform	(0.001)	0.000	0.088
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent	(25.40%)	0.00%	0.00%
Other, net	(0.50%)	(1.50%)	(0.60%)
Effective income tax rate	(5.80%)	(4.60%)	34.30%

INCOME TAXES - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Net operating loss carryforwards	$ 61,057	$ 76,278
Research and development tax credits	32,879	28,664
Deferred revenue	7,868	5,755
Accrued expenses	5,687	9,601
Inventory	4,618	4,906
Stock-based compensation	2,880	1,536
Fixed assets	5,461	7,716
Other temporary differences	2,138	1,943
Deferred tax assets, gross	122,588	136,399
Valuation allowance	(94,980)	(101,484)
Total deferred tax assets	27,608	34,915
Liabilities:
Purchased intangible assets	(22,470)	(26,014)
Unremitted foreign income	(4,827)	(4,487)
Total deferred tax liabilities	(27,297)	(30,501)
Total net deferred tax assets	311	4,414
Deferred income taxes - noncurrent assets	4,959	9,152
Deferred income taxes - noncurrent liabilities	$ (4,648)	$ (4,738)

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 24, 2018	Jan. 01, 2018
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	$ 32,900
Significant change in shareholder ownership, period of change	3 years
Significant change in shareholder ownership, benchmark percentage	50.00%
Decrease in deferred tax assets	$ (4,959)	$ (9,152)
BEAT tax expense			$ 400
Decrease in valuation allowance	500	200
Deferred tax assets, valuation allowance	94,980	101,484
Potential penalties and interest	100	100	200
Interest and penalties accrued in deferred tax liability	700	600	600
Income tax benefit	(7,182)	(3,400)	18,440
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	224,800
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	71,400	82,400
Edgewater
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards acquired				$ 34,000
Tax credit carryforwards acquired				$ 6,000
Deferred tax assets, valuation allowance	200
Income tax benefit		700
GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	11,000
Income tax benefit			$ 16,400
IRELAND \| GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	9,200	$ 9,500
BRAZIL \| GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	$ 2,700
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
Operating Loss Carryforwards [Line Items]
Decrease in deferred tax assets					$ 2,200

INCOME TAXES - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at January 1	$ 3,461	$ 4,528	$ 8,969
Increases related to current year tax positions	292	74	139
Increases related to prior period tax positions	0	122	430
Increases related to business acquisitions	0	0	2,012
Decreases related to prior period tax positions	(821)	(1,263)	(7,022)
Settlements	0	0	0
Unrecognized tax benefits at December 31	$ 2,932	$ 3,461	$ 4,528

MAJOR CUSTOMERS - (Details) - Customer	12 Months Ended
MAJOR CUSTOMERS - (Details) - Customer	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue \| Verizon Communications Inc.
MAJOR CUSTOMERS
Concentration risk, percentage	17.00%	17.00%	17.00%
Revenue \| AT&T Inc.
MAJOR CUSTOMERS
Concentration risk, percentage	12.00%
Accounts receivable
MAJOR CUSTOMERS
Concentration risk, percentage	22.00%	32.00%

RELATED PARTIES (Details) - USD ($)	Apr. 29, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 27, 2017
GENBAND
Related Party Transaction [Line Items]
Promissory note issued to GENBAND equity holders				$ 22,500,000.0
Minimum \| GENBAND
Related Party Transaction [Line Items]
Promissory note interest rate		7.50%
Maximum \| GENBAND
Related Party Transaction [Line Items]
Promissory note interest rate		10.00%
Promissory Note
Related Party Transaction [Line Items]
Promissory note issued to GENBAND equity holders			$ 22,500,000
Promissory note balance			24,100,000
Interest converted to principal			$ 1,600,000
Notes Payable, Other Payables \| Promissory Note
Related Party Transaction [Line Items]
Repayments of outstanding amounts	$ 24,700,000

COMMITMENTS AND CONTINGENCIES - Litigation and Contingencies (Details) $ in Millions		3 Months Ended	12 Months Ended
	Apr. 22, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) case
Loss Contingencies [Line Items]
Ongoing lawsuits \| case			6
Damages awarded from other party	$ 63.0
Cash payments		$ 37.5
Litigation settlement amount awarded	$ 25.5		$ 25.5
Litigation settlement amount awarded, rate	4.00%
Gain from settlement			63.0
Other Current Assets
Loss Contingencies [Line Items]
Litigation settlement amount awarded			8.5
Other Noncurrent Assets
Loss Contingencies [Line Items]
Litigation settlement amount awarded			$ 17.0

QUARTERLY RESULTS (UNAUDITED) - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 161,109	$ 137,653	$ 145,421	$ 118,928	$ 166,896	$ 152,468	$ 137,361	$ 121,180
Cost of revenue	60,164	58,776	64,748	62,339	71,182	70,234	62,250	65,907
Gross profit	100,945	78,877	80,673	56,589	95,714	82,234	75,111	55,273	$ 317,084	$ 308,332	$ 201,496
(Loss) income from operations	(148,822)	2,686	(7,096)	(36,228)	1,177	(7,566)	(16,636)	(42,383)	(189,460)	(65,408)	(55,229)
Net loss	$ (150,363)	$ 1,650	$ 49,470	$ (30,832)	$ (1,826)	$ (10,158)	$ (19,922)	$ (44,904)	$ (130,075)	$ (76,810)	$ (35,252)
Loss per share:
Basic (in dollars per share)	$ (1.36)	$ 0.01	$ 0.45	$ (0.29)	$ (0.02)	$ (0.10)	$ (0.20)	$ (0.44)	$ (1.19)	$ (0.74)	$ (0.60)
Diluted (in dollars per share)	$ (1.36)	$ 0.01	$ 0.45	$ (0.29)	$ (0.02)	$ (0.10)	$ (0.20)	$ (0.44)	$ (1.19)	$ (0.74)	$ (0.60)
Shares used in computing loss per share:
Basic (in shares)	110,269	110,080	110,394	108,167	106,607	104,918	102,160	101,917	109,734	103,916	58,822
Diluted (in shares)	110,269	110,756	110,698	108,167	106,607	104,918	102,160	101,917	109,734	103,916	58,822