Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
Document and Entity Information - USD ($) $ in Thousands	Dec. 31, 2018	Feb. 25, 2019	Jun. 29, 2018
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, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		107,270,854
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 363,563

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 43,694	$ 57,073
Marketable securities	7,284	17,224
Accounts receivable, net	187,853	165,156
Inventory	22,602	21,303
Other current assets	17,002	21,463
Total current assets	278,435	282,219
Property and equipment, net	27,042	24,780
Intangible assets, net	251,391	244,414
Goodwill	383,655	335,716
Investments	0	9,031
Deferred income taxes	9,152	8,434
Other assets	7,484	6,289
Total assets	957,159	910,883
Current liabilities:
Revolving credit facility	55,000	20,000
Accounts payable	45,304	45,851
Accrued expenses and other	84,263	76,380
Deferred revenue	105,087	100,571
Total current liabilities	289,654	242,802
Long-term debt, related party	24,100	22,500
Deferred revenue, net of current	17,572	14,184
Deferred income taxes	4,738	2,787
Other long-term liabilities	30,797	13,189
Total liabilities	366,861	295,462
Commitments and contingencies
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, 106,815,636 shares issued and outstanding at December 31, 2018; 101,752,856 shares issued and outstanding at December 31, 2017	11	10
Additional paid-in capital	1,723,576	1,684,768
Accumulated deficit	(1,136,992)	(1,072,426)
Accumulated other comprehensive income	3,703	3,069
Total stockholders' equity	590,298	615,421
Total liabilities and stockholders' equity	$ 957,159	$ 910,883

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	240,000,000	240,000,000
Common stock, shares issued	106,815,636	101,752,856
Common stock, shares outstanding	106,815,636	101,752,856

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Total revenue	$ 577,905	$ 329,942	$ 252,591
Cost of revenue:
Total cost of revenue	269,573	128,446	84,980
Gross profit	308,332	201,496	167,611
Operating expenses:
Research and development	145,462	101,481	72,841
Sales and marketing	128,276	83,403	68,539
General and administrative	66,036	47,642	35,948
Acquisition- and integration-related	16,951	14,763	1,152
Restructuring	17,015	9,436	2,740
Total operating expenses	373,740	256,725	181,220
Loss from operations	(65,408)	(55,229)	(13,609)
Interest (expense) income, net	(4,230)	263	769
Other (expense) income, net	(3,772)	1,274	1,424
Loss before income taxes	(73,410)	(53,692)	(11,416)
Income tax (provision) benefit	(3,400)	18,440	(2,516)
Net loss	$ (76,810)	$ (35,252)	$ (13,932)
Loss per share:
Basic (in dollars per share)	$ (0.74)	$ (0.60)	$ (0.28)
Diluted (in dollars per share)	$ (0.74)	$ (0.60)	$ (0.28)
Shares used to compute loss per share:
Basic (in shares)	103,916	58,822	49,385
Diluted (in shares)	103,916	58,822	49,385
Product
Revenue:
Total revenue	$ 279,014	$ 181,119	$ 146,381
Cost of revenue:
Total cost of revenue	142,185	70,250	47,367
Service
Revenue:
Total revenue	298,891	148,823	106,210
Cost of revenue:
Total cost of revenue	$ 127,388	$ 58,196	$ 37,613

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (76,810)	$ (35,252)	$ (13,932)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	220	(1,940)	54
Unrealized gain on available-for-sale marketable securities, net of reclassification adjustments for realized amounts	45	146	51
Employee retirement benefits	369	(578)	0
Other comprehensive income (loss), net of tax	634	(2,372)	105
Comprehensive loss, net of tax	$ (76,176)	$ (37,624)	$ (13,827)

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
Balance at Dec. 31, 2015	$ 223,026	$ 49	$ 1,240,803	$ (1,023,242)	$ 5,416
Balance (in shares) at Dec. 31, 2015		49,473,789
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	1,360		1,360
Issuance of common stock in connection with employee stock purchase plan (in shares)		225,031
Exercise of stock options	153		153
Exercise of stock options (in shares)		23,070
Vesting of restricted stock awards and units	0	$ 1	(1)
Vesting of restricted stock awards and units (in shares)		792,773
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		18,438
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(1,810)		(1,810)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(231,620)
Repurchase of common stock	(9,530)	$ (1)	(9,529)
Repurchase of common stock (in shares)		(1,259,600)
Stock-based compensation expense	19,768		19,768
Other comprehensive income (loss)	87				87
Net loss	(13,932)			(13,932)
Balance at Dec. 31, 2016	219,122	$ 49	1,250,744	(1,037,174)	5,503
Balance (in shares) at Dec. 31, 2016		49,041,881
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with employee stock purchase plan	1,252		1,252
Issuance of common stock in connection with employee stock purchase plan (in shares)		249,621
Exercise of stock options	617		617
Exercise of stock options (in shares)		105,688
Vesting of restricted stock awards and units	0
Vesting of restricted stock awards and units (in shares)		2,160,553
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		145,357
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(7,523)		(7,523)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(807,952)
Shares issued as consideration in connection with acquisition						$ 413,982	$ 5	$ 413,977
Shares issued as consideration in connection with acquisition (in shares)						50,857,708
Stock-based compensation expense	25,657		25,657
Reclassification between Common stock and Additional paid-in capital to record change in par value of common stock	0	$ (44)	44
Other comprehensive income (loss)	(2,434)				(2,434)
Net loss	(35,252)			(35,252)
Balance at Dec. 31, 2017	$ 615,421	$ 10	1,684,768	(1,072,426)	3,069
Balance (in shares) at Dec. 31, 2017	101,752,856	101,752,856
Increase (Decrease) in Stockholders' Equity
Exercise of stock options	$ 73		73
Exercise of stock options (in shares)		15,935
Vesting of restricted stock awards and units	0
Vesting of restricted stock awards and units (in shares)		1,278,062
Vesting of performance-based stock awards	0
Vesting of performance-based stock awards (in shares)		57,768
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations	(2,024)		(2,024)
Shares of restricted stock returned to the Company under net share settlements to satisfy tax withholding obligations (in shares)		(524,516)
Shares issued as consideration in connection with acquisition						$ 30,000	$ 1	29,999
Shares issued as consideration in connection with acquisition (in shares)							4,235,531
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 at Dec. 31, 2018	$ 590,298	$ 11	$ 1,723,576	$ (1,136,992)	$ 3,703
Balance (in shares) at Dec. 31, 2018	106,815,636	106,815,636

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (76,810)	$ (35,252)	$ (13,932)
Adjustments to reconcile net loss to cash flows (used in) provided by operating activities:
Depreciation and amortization of property and equipment	11,200	8,486	7,970
Amortization of intangible assets	49,723	17,112	7,500
Stock-based compensation	11,072	25,657	19,768
Impairment of intangible assets	0	5,471	0
Deferred income taxes	513	(20,361)	1,088
Other	4,611	(1,340)	(1,265)
Changes in operating assets and liabilities:
Accounts receivable	(13,017)	(30,759)	(851)
Inventory	993	5,786	4,858
Other operating assets	5,036	269	506
Accounts payable	(6,057)	13,415	(821)
Accrued expenses and other long-term liabilities	(13,422)	(4,263)	(7,778)
Deferred revenue	16,563	23,859	2,149
Net cash (used in) provided by operating activities	(9,595)	8,080	19,192
Cash flows from investing activities:
Purchases of property and equipment	(7,907)	(3,999)	(4,626)
Business acquisitions, net of cash acquired	(46,389)	(42,951)	(20,669)
Purchases of marketable securities	0	(28,731)	(78,528)
Sale/maturities of marketable securities	18,919	96,112	75,178
Proceeds from the sale of intangible assets	0	576	1,298
Net cash (used in) provided by investing activities	(35,377)	21,007	(27,347)
Cash flows from financing activities:
Borrowings under revolving line of credit	197,500	15,500	0
Principal payments on revolving line of credit	(162,500)	(13,500)	0
Principal payments of capital lease obligations	(652)	(99)	(43)
Payment of debt issuance costs	(624)	(731)	0
Proceeds from the sale of common stock in connection with employee stock purchase plan	0	1,252	1,360
Proceeds from the exercise of stock options	73	617	153
Payment of tax withholding obligations related to net share settlements of restricted stock awards	(2,024)	(7,523)	(1,810)
Repurchase of common stock	0	0	(9,530)
Net cash provided by (used in) financing activities	31,773	(4,484)	(9,870)
Effect of exchange rate changes on cash and cash equivalents	(180)	547	(163)
Net (decrease) increase in cash and cash equivalents	(13,379)	25,150	(18,188)
Cash and cash equivalents, beginning of year	57,073	31,923	50,111
Cash and cash equivalents, end of period	43,694	57,073	31,923
Supplemental disclosure of cash flow information:
Interest paid	2,367	317	41
Income taxes paid	5,505	2,290	1,249
Income tax refunds received	537	274	511
Supplemental disclosure of non-cash investing activities:
Capital expenditures incurred, but not yet paid	1,127	1,043	277
Property and equipment acquired under capital lease	2,178	0	36
Business acquisition purchase consideration - common stock issued	30,000	413,982	0
Business acquisition purchase consideration - deferred payments	30,000	0	0
Business acquisition purchase consideration - note issued to selling equity holders	0	22,500	0
Business acquisition purchase consideration - assumed equity awards	747	0	0
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	$ 8,312	$ 20,515	$ 10,376

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2018
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 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 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. On September 26, 2016 (the "Taqua Acquisition Date"), the Company acquired Taqua, LLC ("Taqua"). The financial results of Taqua are included in the Company's consolidated financial statements beginning on the Taqua Acquisition 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, not 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 generally 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 vs the total estimated labor time required to complete the performance obligation. Revenue is recorded proportionally as costs are incurred or 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 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 remaining 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. Restricted Cash The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Restricted cash is recorded within other current assets on the consolidated balance sheet. 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. Realized and unrealized foreign currency 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 a net foreign currency loss of $4.6 million for the year ended December 31, 2018, a net foreign currency gain of $0.7 million for the year ended December 31, 2017 and a net foreign currency loss of $0.3 million for the year ended December 31, 2016. 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 comprised of certain intangible assets arising from the Merger, the Edgewater Acquisition and previous acquisitions. 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. The Company performed its step one assessments for each of the years ended December 31, 2018, 2017 and 2016 and determined each year that its fair value was in excess of its carrying value and accordingly, there was no impairment of goodwill. At certain times during the year ended December 31, 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 has concluded that the fair value exceeds carrying value at that date, the Company regularly monitors for changes in circumstances, including changes to the Company's performance, that could result in impairment of goodwill. Other Assets Other assets are primarily comprised of the long-term portions of inventory, prepaid expenses and deposits. 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. Research and Development Costs Research and development costs are expensed as incurred. 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 both December 31, 2018 and 2017. 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, 2018, $0.3 million for the year ended December 31, 2017 and $0.1 million for the year ended December 31, 2016. 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 maker is its President and Chief Executive Officer. 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. In certain cases, Ribbon has been 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 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, 2018, 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 $4.5 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2018. 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 In May 2017, the Financial Accounting Standards Board ("FASB") issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"), which amends the scope of modification accounting for share-based payment arrangements such that an entity would not apply modification accounting if the fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. ASU 2017-09 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The adoption of ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In March 2017, the FASB issued ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ("ASU 2017-07"). ASU 2017-07 amends the requirements in ASC 715, Compensation - Retirement Benefits ("ASC 715") to require entities to disaggregate the current-service-cost component from the other components of net benefit cost (the "other components") and include it with other current compensation costs for related employees, present the other components elsewhere in the income statement and outside of income from operations if such a subtotal is presented and disclose the income statement lines that contain the other components if they are not presented on appropriately described separate lines. ASU 2017-07 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The adoption of ASU 2017-07 did not have a material impact on the Company's consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), which adds or clarifies guidance on eight cash flow issues, including debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or certain other debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees, beneficial interests in securitization transactions and separately identifiable cash flows and application of the predominance principle. ASU 2016-15 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. Entities must apply the guidance retrospectively to all periods presented but may apply it prospectively from the earliest date practicable if retrospective application would be impracticable. The adoption of ASU 2016-15 did not have a material impact on the Company's 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 is effective for the Company beginning January 1, 2020. The Company is currently assessing the potential impact of the adoption of ASU 2018-15 on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Section A - Leases: Amendments to the FASB Accounting Standards Codification ("ASU 2016-02"), its new standard on accounting for leases. ASU 2016-02 introduces a lessee model that brings most leases onto the balance sheet. ASU 2016-02 eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. ASU 2016-02 became effective for the Company for both interim and annual periods beginning January 1, 2019. Upon adoption of ASU 2016-02, the Company will recognize lease obligations for the right to use these assets in connection with its existing lease agreements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements ("ASU 2018-11") and ASU 2018-10, Codification Improvements to Topic 842, Leases, both of which provided improvements to certain aspects of the guidance in ASC 842, Leases. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842, which provided additional clarification and implementation guidance. The Company has elected to use the alternative transition method as described in ASU 2018-11, which allows entities to initially apply ASU 2016-02 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings, with no subsequent adjustments to prior period lease costs for comparability. To illustrate the magnitude of this change, the amount of the Company's off-balance sheet operating leases at December 31, 2018 is disclosed in Note 22. Beginning on January 1, 2019, the Company's operating leases, excluding those with terms less than 12 months, were discounted and recorded as assets and liabilities in the Company's consolidated balance sheet. 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 2018-14, 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 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. ASU 2018-14 is 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 (“ASC 820”). ASU 2018-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting. In July 2018, the FASB issued ASU 2018-09, Codification Improvements (“ASU 2018-09”), which contains amendments to clarify, correct errors in or make minor improvements to the 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. ASU 2018-09 became effective for the Company beginning January 1, 2019. In June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"), 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. ASU 2018-07 became effective for the Company beginning January 1, 2019. 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 ("ASU 2018-02"), 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. ASU 2018-02 became effective for the Company beginning January 1, 2019. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ("ASU 2016-16"), 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. ASU 2016-16 became effective for the Company beginning January 1, 2019 for both interim and annual reporting periods. 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. ASU 2016-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting periods, with early adoption permitted.

NATURE OF THE BUSINESS	12 Months Ended
NATURE OF THE BUSINESS	Dec. 31, 2018
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.

BUSINESS ACQUISITONS

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Business Acquisitions

BUSINESS ACQUISITIONS

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 is included as a component of Accrued expenses and other, and the noncurrent portion is 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 will be payable on March 8, 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, 2018, the valuation of acquired assets, identifiable intangible assets and certain assumed liabilities was preliminary. During the fourth quarter of 2018, the Company recorded changes to the initial preliminary purchase price allocation, primarily comprised of a decrease of $0.7 million to current assets and an increase to other current liabilities of $0.5 million. These adjustments, along with other immaterial adjustments, resulted in a net increase to goodwill of $1.3 million since September 30, 2018. The Company is continuing the process of investigating the facts and circumstances existing as of the Edgewater Acquisition Date in order to finalize its valuation. The Company expects to finalize the valuation of the assets acquired and liabilities assumed by the third quarter of 2019.

A summary of the preliminary 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 8.4 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 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.

NewCo assumed the liability under GENBAND's revolving credit facility with Silicon Valley Bank, 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 its 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 years ended December 31, 2017 and 2016 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 and inclusion of such costs in the year ended December 31, 2016.

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			2016
	(unaudited)
Revenue	$	615,286		$	631,914
Net loss	$	(69,741	)	$	(147,394	)
Loss per share	$	(0.69	)	$	(1.46	)

Taqua, LLC

The Company acquired Taqua, a privately-held company, on September 26, 2016 (the "Taqua Acquisition Date"). Taqua enables the transformation of software-based service provider networks to deliver next-generation voice, video and messaging services, including VoIP, VoWiFi and VoLTE. In consideration for the acquisition of Taqua, Sonus paid $19.9 million in cash to the sellers on the Taqua Acquisition Date, net of cash acquired. The Company also entered into an Earn-Out Agreement, dated as of September 26, 2016, with Taqua Holdings, LLC and Jeffrey L. Brawner, the seller representative in the transaction, under which there is the potential for additional cash payments of up to $65 million in the aggregate to the sellers if certain annual revenue thresholds are exceeded as measured annually through 2020. The Company had initially recorded $10 million of contingent consideration as of the Taqua Acquisition Date, with the estimate based on historical sales and probability weighted cash flows related to forecasted sales. Because there are unobservable inputs to the valuation methodology that are significant to the measurement of its fair value, namely, forecasted sales, the Company had categorized the earn-out at Level 3 within the fair value hierarchy. During the fourth quarter of 2016, the Company reassessed the historical and updated forecasted sales and accordingly, reversed the previous estimated contingent consideration such that as of December 31, 2016, no incremental contingent consideration was recorded. In July 2017, Taqua Holdings, LLC ("Holdings") filed a lawsuit against the Company alleging that the Company had breached the Earn-Out Agreement. Following mediation, in April 2018, the Company and Holdings entered into a settlement agreement pursuant to which the Company agreed to release the $0.6 million of remaining funds held in escrow and to pay an additional $1.4 million to Holdings in lieu of any future payments under the Earn-Out Agreement (the "Taqua Settlement Agreement"). The Company paid the total amount due under the Taqua Settlement Agreement of $2.0 million in April 2018.

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

The Company finalized its valuation of the identifiable intangible assets in the second quarter of 2017. During the first quarter of 2017 and the fourth quarter of 2016, the Company recorded changes to the initial preliminary purchase price allocation. The primary adjustments in the first quarter of 2017 were a $0.4 million increase to current liabilities and a $0.1 million increase to noncurrent liabilities. During the fourth quarter of 2016, the Company recorded changes to the initial preliminary purchase price allocation. The primary adjustments recorded in the fourth quarter of 2016 were the aforementioned reversal of the $10 million of previously recorded contingent consideration, a reduction of $12.1 million to the developed technology intangible asset and an increase of $5.5 million to the customer relationship intangible asset. These adjustments, as well as other immaterial adjustments to other balance sheet accounts, resulted in a net reduction to goodwill of $2.2 million. Based on this final purchase price allocation, the Company recorded $9.6 million of goodwill, which is primarily due to expected synergies between the combined companies and expanded market opportunities resulting from the expanded product offering portfolio. The goodwill is deductible for tax purposes.

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



Fair value of consideration transferred:
Cash, net of cash acquired	$	19,919

Fair value of assets acquired and liabilities assumed:
Current assets	$	3,347
Property and equipment	1,478
Intangible assets:
Developed technology	2,100
Customer relationships	9,510
Goodwill	9,581
Other noncurrent assets	23
Current liabilities	(5,435		)
Long-term liabilities	(685		)
	$	19,919

The Company's revenue for the year ended December 31, 2016 included $1.9 million of revenue attributable to Taqua since the Taqua Acquisition Date. The inclusion of Taqua's operations for the period from the Taqua Acquisition Date to December 31, 2016 in the Company's financial results for the year ended December 31, 2016 increased the Company's loss by $4.7 million. The Company has not provided pro forma financial information, as the historical amounts were not significant to the Company's consolidated financial statements.

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- and integration-related expenses 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 amount recorded in the year ended December 31, 2016 related to professional fees in connection with the acquisition of Taqua.

The components of acquisition- and integration-related costs incurred in the years ended December 31, 2018, 2017 and 2016 are were as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Professional and services fees (acquisition-related)	$	7,627	$	11,916	$	1,152
Management bonuses (acquisition-related)	1,972		931		—
Integration-related expenses	7,352		1,916		—
	$	16,951	$	14,763	$	1,152

EARNINGS (LOSS) PER SHARE

12 Months Ended

Dec. 31, 2018

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,
	2018	2017	2016
Weighted average shares outstanding—basic	103,916	58,822	49,385
Potential dilutive common shares	—	—	—
Weighted average shares outstanding—diluted	103,916	58,822	49,385

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, 2.5 million shares for the year ended December 31, 2017 and 8.0 million shares for the year ended December 31, 2016 have not been included in the computation of diluted loss per share because their effect would have been antidilutive.

CASH EQUIVALENTS AND INVESTMENTS

12 Months Ended

Dec. 31, 2018

Investments, All Other Investments [Abstract]

CASH EQUIVALENTS, MARKETABLE SECURITIES AND INVESTMENTS

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.

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 year ended December 31, 2016, the Company sold $4.9 million of its available-for-sale securities. The Company recognized nominal gross gains and losses from the sales of these securities.

Investments with continuous unrealized losses for one year or greater at December 31, 2018 were nominal; however, since the Company does not intend to sell these securities and does not believe it will be required to sell any securities before they recover in value, it does not believe these declines are other-than-temporary.

On a quarterly basis, the Company reviews its investments to determine if there have been any events that could create a credit impairment. Based on its reviews, the Company does not believe that any impairment existed with its current holdings at December 31, 2018.

The amortized cost, gross unrealized gains and losses and fair value of the Company's cash equivalents and investments at December 31, 2018 and 2017 were comprised of the following (in thousands):



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

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



	December 31, 2017
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	1,254	$	—	$	—		$	1,254

Short-term investments
U.S. government agency notes	$	4,091	$	—	$	(19	)	$	4,072
Corporate debt securities	8,048		—		(31		)	8,017
Certificates of deposit	5,135		—		—			5,135
	$	17,274	$	—	$	(50	)	$	17,224
Investments
U.S. government agency notes	$	3,992	$	—	$	(28	)	$	3,964
Corporate debt securities	3,908		—		(24		)	3,884
Certificates of deposit	1,183		—		—			1,183
	$	9,083	$	—	$	(52	)	$	9,031

The Company's available-for-sale debt securities that are classified as Investments in the consolidated balance sheet mature after one year but within two years or less from the balance sheet date. At December 31, 2018, the Company did not hold any investments that matured beyond one year.

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 and 2017. These financial assets are comprised of the Company's available-for-sale debt securities and reported under the captions Cash and cash equivalents, Short-term investments and Investments 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	$	—	$	—

Short-term investments
U.S. government agency notes	$	3,989	$	—	$	3,989	$	—
Corporate debt securities	3,295		—		3,295		—
	$	7,284	$	—	$	7,284	$	—



			Fair value measurements at December 31, 2017 using:
	Total carrying value at December 31, 2017		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	1,254	$	1,254	$	—	$	—

Short-term investments
U.S. government agency notes	$	4,072	$	—	$	4,072	$	—
Corporate debt securities	8,017		—		8,017		—
Certificates of deposit	5,135		—		5,135		—
	$	17,224	$	—	$	17,224	$	—
Investments
U.S. government agency notes	$	3,964	$	—	$	3,964	$	—
Corporate debt securities	3,884		—		3,884		—
Certificates of deposit	1,183		—		1,183		—
	$	9,031	$	—	$	9,031	$	—

The Company's marketable securities and investments have been 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, 2018

Receivables [Abstract]

ACCOUNTS RECEIVABLE, NET

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



	December 31,
	2018			2017
Accounts receivable	$	188,522		$	165,229
Allowance for doubtful accounts	(669		)	(73		)
Accounts receivable, net	$	187,853		$	165,156

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
2018	$	73	$	351	$	620		$	(375	)	$	669
2017	$	10	$	154	$	(56	)	$	(35	)	$	73
2016	$	10	$	10	$	—		$	(10	)	$	10

- Definition

The entire disclosure for claims held for amounts due a entity, excluding financing receivables. Examples include, but are not limited to, trade accounts receivables, notes receivables, loans receivables. Includes disclosure for allowance for credit losses.

+ References + Details

PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT

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



		December 31,
	Useful Life	2018			2017
Equipment	2-5 years	$	76,423		$	67,415
Software	2-5 years	24,707			21,977
Furniture and fixtures	3-5 years	1,490			1,892
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	21,220			18,428
		123,840			109,712
Less accumulated depreciation and amortization		(96,798		)	(84,932		)
Property and equipment, net		$	27,042		$	24,780

The Company recorded depreciation and amortization expense related to property and equipment of $11.2 million for the year ended December 31, 2018, $8.5 million for the year ended December 31, 2017 and $8.0 million for the year ended December 31, 2016. During each of the years ended December 31, 2018, 2017 and 2016, 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 capital leases included in the amounts above were as follows (in thousands):



	December 31,
	2018			2017
Cost	$	2,979		$	664
Less accumulated depreciation	(875		)	(180		)
Property and equipment under capital leases, net	$	2,104		$	484

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



	December 31,
	2018		2017
United States	$	17,862	$	17,576
Canada	4,076		1,740
Asia/Pacific	3,841		3,853
Europe	1,100		1,400
Other	163		211
	$	27,042	$	24,780

INTANGIBLE ASSETS AND GOODWILL

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

INTANGIBLE ASSETS AND GOODWILL

The Company's intangible assets at December 31, 2018 and 2017 consisted of the following (in thousands):



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



December 31, 2017	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.90	153,380		24,211		129,169
Customer relationships	9.32	120,840		12,015		108,825
Trade names	3.00	900		80		820
Internal use software	3.00	730		730		—
	7.77	$	281,450	$	37,036	$	244,414

* 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, 2018, 2017 and 2016 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2018		2017		2016		Statement of operations classification
Developed technology	$	38,976	$	18,358	$	6,038	Cost of revenue - product
Customer relationships	10,203		4,145		1,462		Sales and marketing
Trade names	544		80		—		Sales and marketing
	$	49,723	$	22,583	$	7,500

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 statements of operations for the year ended December 31, 2017.

Estimated future amortization expense for the Company's intangible assets at December 31, 2018 was as follows (in thousands):



Years ending December 31,
2019	$	47,411
2020	46,552
2021	40,571
2022	34,156
2023	26,756
Thereafter	55,945
	$	251,391

Goodwill is recorded when the consideration for an acquisition exceeds the fair value of net tangible and identifiable intangible assets acquired. The changes in the carrying value of the Company's goodwill in the years ended December 31, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2018			2017
Balance at January 1
Goodwill	$	338,822		$	52,499
Accumulated impairment losses	(3,106		)	(3,106		)
	335,716			49,393
Acquisition of Edgewater	48,053			—
Acquisition of GENBAND	—			285,825
Acquisition of Taqua and subsequent purchase accounting adjustments	—			498
Write-off of goodwill attributable to dissolved subsidiary	(114		)	—
Balance at December 31	$	383,655		$	335,716

RESTRUCTURING ACCRUALS

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

RESTRUCTURING ACCRUALS

The Company recorded restructuring expense aggregating $17.0 million in the year ended December 31, 2018, $9.4 million in the year ended December 31, 2017 and $2.7 million in the year ended December 31, 2016.

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 $8.5 million of restructuring expense in 2017 for severance and related costs for approximately 120 employees. The Company recorded $16.1 million of additional restructuring 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. as it continues to combine the two businesses and benefit from operational synergies. The Company believes that the severance payments for the currently recorded accruals will be paid in 2019 and that the payments related to restructured facilities will be completed in 2029, when the last of the leases for these restructured facilities expires. The Company anticipates it will record additional future expense in connection with this initiative for headcount and redundant facilities aggregating approximately $5 million.

Summaries of the Merger Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	—	$	8,508	$	—	$	(913	)	$	7,595

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. The Company recorded $0.9 million of restructuring expense in 2018 and a credit of $0.3 million credit to restructuring 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 Company does not expect to record additional expense in connection with this initiative with the exception of any additional adjustments for changes in estimated costs. The Company expects that the payments related to this assumed liability will be completed in 2020.

Summaries of the GENBAND Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



Year ended December 31, 2018	Balance at January 1, 2018		Liability assumed in connection with Merger		Initiatives charged to expense		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



Year ended December 31, 2017	Balance at January 1, 2017		Liability assumed in connection with Merger		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	—	$	3,663	$	—	$	(158	)	$	(1,589	)	$	1,916
Facilities	—		431		—		(123		)	(103		)	205
	$	—	$	4,094	$	—	$	(281	)	$	(1,692	)	$	2,121

2016 Restructuring Initiative

In July 2016, the Company announced a program (the "2016 Restructuring Initiative") to further accelerate its investment in new technologies, as the communications industry migrates to a cloud-based architecture and as the Company pursues new 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 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 have been implemented and accordingly, the Company does not expect to record additional expense in connection with this initiative. The amounts accrued for severance and related costs had been fully paid by the end of the third quarter of 2017. The Company expects that the amounts accrued for facilities will be paid by the end of October 2019, when the lease on the Rochester Facility expires.

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. The Company recorded $1.5 million of expense in the year ended December 31, 2016 related to headcount reductions.

Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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
Facilities	$	95	$	—	$	—	$	(37	)	$	58



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	497	$	405	$	(26	)	$	(876	)	$	—
Facilities	—		126		—			(31		)	95
	$	497	$	531	$	(26	)	$	(907	)	$	95

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 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. The actions under the Taqua Restructuring Initiative have been implemented and accordingly, the Company does not expect to record additional expense in connection with this initiative. The amounts accrued for severance and related costs had been fully paid by the end of the third quarter of 2017. The Company expects that the amounts accrued for facilities will be paid by the end of 2019.

In connection with the Taqua Restructuring Initiative, the Company recorded $0.7 million of restructuring expense, including adjustments for changes in estimated costs, in the year ended December 31, 2017, comprised of $0.2 million for severance and related costs and $0.5 million related to redundant facilities. The Company recorded $1.2 million of restructuring expense for this initiative in the year ended December 31, 2016, comprised of $1.0 million for severance and related costs and $0.2 million related to redundant facilities.

Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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
Facilities	$	365	$	—	$	—	$	(332	)	$	33



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	384	$	245	$	(49	)	$	(580	)	$	—
Facilities	218		508		—			(361		)	365
	$	602	$	753	$	(49	)	$	(941	)	$	365

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 $0.5 million at December 31, 2018 and $0.2 million at December 31, 2017. These amounts represent future lease payments on restructured facilities.

DEBT	12 Months Ended
DEBT	Dec. 31, 2018
Debt Disclosure [Abstract]
DEBT	DEBT Proposed Offering of Convertible Senior Note On November 13, 2018, the Company announced that it intended to offer, subject to market conditions and other factors, $150.0 million aggregate principal amount of convertible senior notes due 2023 in a private offering to qualified institutional buyers pursuant to Rule 144A under the Securities Act of 1933, as amended. The Company expected to grant the initial purchasers a 30-day option to purchase up to an additional $25.0 million aggregate principal amount of such notes, solely to cover over-allotments, if any. On that same day, the Company decided not to proceed with its previously-announced offering and consequently did not sell any securities as part of such offering. The Company believed that then-current market conditions were not conducive for an offering on terms that would be in the best interests of the Company's stockholders. In connection with this offering, the Company incurred $1.0 million of expense, primarily for professional fees, which is included as a component of General and administrative expense in the Company's consolidated statement of operations for the year ended December 31, 2018. Assumed Senior Secured Credit Agreement On the Merger Date and in connection with the Merger, the Company assumed GENBAND's Senior Secured Credit Agreement (the "Prior Credit Agreement" with Silicon Valley Bank ("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 Prior Credit Agreement with SVB effective July 1, 2016, with two of its operating subsidiaries as borrowers and GENBAND as the guarantor. The Prior 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 Prior Credit Agreement was superseded by a Senior Secured Credit Facilities Credit Agreement, as amended, which was entered into on December 21, 2017 and is discussed below. Senior Secured Credit Facility On December 21, 2017, the Company entered into a Senior Secured Credit Facilities Credit Agreement (as amended, the “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 Prior Credit Agreement. The Credit Facility includes $100 million of commitments, the full amount of which is available for revolving loans, a $15 million sublimit that is available for letters of credit and a $15 million sublimit that is available for swingline loans. The Credit Facility is scheduled to mature in December 2021, subject to a springing maturity if, on or before July 14, 2020, the existing promissory note issued to certain shareholders is not converted or extended to March 2022 or later. The Credit Facility contains 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 Credit Facility. On June 24, 2018, the Company amended the Credit Facility to, among other things, permit the Edgewater Acquisition and related transactions. The indebtedness and other obligations under the 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 Credit Facility is secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including the Company. The Credit Facility requires periodic interest payments on outstanding borrowings until maturity. The Borrower may prepay all revolving loans under the Credit Facility at any time without premium or penalty (other than customary LIBOR breakage costs), subject to certain notice requirements. Revolving loans under the Credit Facility bear 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 varies depending on the Company’s consolidated leverage ratio (as defined in the Credit Facility). The base rate and the LIBOR rate are each subject to a zero percent floor. The Borrower is 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 Credit Facility. Additionally, with respect to all letters of credit outstanding under the Credit Facility, the Borrower is 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 Credit Facility requires compliance with certain financial covenants, including a minimum consolidated quick ratio, minimum consolidated interest coverage ratio and maximum consolidated leverage ratio, all of which are defined in the Credit Facility and tested on a quarterly basis. In addition, the 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, 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 Credit Facility as of December 31, 2018 and 2017. The 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 Credit Facility will immediately become due and payable. If any other event of default exists under the Credit Facility, the lenders may accelerate the maturity of the obligations outstanding under the 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 Credit Facility, the lenders may commence foreclosure or other actions against the collateral. If any default exists under the Credit Facility, or if the Borrower is unable to make any of the representations and warranties as stated in the Credit Facility at the applicable time, the Borrower will be unable to borrow funds or have letters of credit issued under the 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 Credit Facility. At December 31, 2017, the Company had an outstanding debt balance of $20 million at an interest rate of 4.51% and $2.9 million of outstanding letters of credit at an interest rate of 2.00% under the Credit Facility. 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 equity holders (the "Promissory Note"). The Promissory Note does not amortize and the principal thereon is payable in full on the third anniversary of its execution. Interest on the Promissory Note is 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, constitutes an event of default under the Promissory Note. If an event of default occurs under the Promissory Note, the payees may 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 is not paid on the interest payment date will 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.

REVENUE RECOGNITION

12 Months Ended

Dec. 31, 2018

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, the Company concluded it did not have 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 amortizing 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 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, 2018, 2017 and 2016 was disaggregated geographically as follows:



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, 2016	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	100,589	$	60,760	$	12,630	$	173,979
Europe, Middle East and Africa	21,073		10,769		2,021		33,863
Japan	12,258		9,831		3,524		25,613
Other Asia Pacific	8,226		3,478		827		12,531
Other	4,235		2,157		213		6,605
	$	146,381	$	86,995	$	19,215	$	252,591

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, 2018, 2017 and 2016 was as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Indirect sales through channel program	$	69,232	$	43,138	$	38,204
Direct sales	209,782		137,981		108,177
	$	279,014	$	181,119	$	146,381

The Company's product revenue from sales to enterprise customers and from sales to service provider customers for the years ended December 31, 2018, 2017 and 2016 was as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Sales to enterprise customers	$	57,534	$	35,592	$	28,508
Sales to service provider customers	221,480		145,527		117,873
	$	279,014	$	181,119	$	146,381

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 year ended December 31, 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 year ended December 31, 2018 were as follows (in thousands):



	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 increase in accounts receivable was primarily due to the Edgewater Acquisition. The increase in deferred revenue was primarily due to the Edgewater Acquisition. The Company recognized 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, 2018, the Company expects that approximately $12 million will be recognized as revenue in 2020, approximately $5 million will be recognized as revenue in 2021 and approximately $1 million will be recognized as revenue in 2022 and beyond.

The Company elected to adopt the practical expedient related to the presentation of shipping and handling costs. Accordingly, all freight-related customer invoicing is being recorded as revenue, while the shipping and handling costs that occur after control of the promised goods or services transfer to the customer are being reported as fulfillment costs as 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, 2018, the Company had $2.7 million of deferred sales commissions capitalized.

Adoption of ASC 606

Under the modified retrospective method, the Company applied ASC 606 to those contracts which 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, Revenue Recognition ("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.

STOCK-BASED COMPENSATION PLANS

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCK-BASED COMPENSATION PLANS

Amended and Restated Stock Incentive Plan

The Company's Amended and Restated Stock Incentive Plan (the "Plan"), provides for the award of options to purchase the Company's common stock ("stock options"), stock appreciation rights ("SARs"), restricted stock awards ("RSAs"), restricted stock units ("RSUs"), performance-based stock awards ("PSAs"), performance-based stock units ("PSUs") and other stock-based awards to employees, officers, directors (including those directors who are not employees or officers of the Company), consultants and advisors of the Company and its subsidiaries. In connection with the Merger, the Company assumed the Plan with all of its then-current terms and conditions.

At December 31, 2018, there were 3.5 million shares available for future issuance under the 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 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 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 Plan. Accordingly, at December 31, 2018, there were no shares available for future issuance under the 2012 Plan.

2008 Stock Incentive Plan

In connection with the acquisition of 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 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 Plan. Accordingly, at December 31, 2018 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 outstanding stock options and certain outstanding full value awards. 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 will continue to be outstanding until they are either exercised or expire.

RSAs and RSUs - as prescribed by the Company's Plan, any unvested RSAs and RSUs that were scheduled to vest within one year from the Merger Date became 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 of Directors 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 former 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 would continue 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 will be determined by the Compensation Committee of the Board of Directors (the "Compensation Committee"). The Company expects to grant the 2018 Bonus Shares and Uplift Shares on a date concurrent with the timing of the payout of the cash portion of the 2018 Bonus, if any is earned. The 2018 Bonus Shares and Uplift Shares will be fully vested on the grant date; however, each executive is restricted from trading the 2018 Bonus Shares and Uplift Shares for five months after the date of grant.

The number of shares of the Company’s common stock that will be granted to those executives who elected to participate in the Stock Bonus Program but are not eligible for the Uplift Shares will be calculated by multiplying the amount of their 2018 Bonus by their individual 2018 Bonus Shares percentage election, divided by the closing price of the Company’s common stock on the date of grant. The number of shares of the Company’s common stock that will be granted to those executives who elected to participate in the Stock Bonus Program and are eligible for the Uplift Shares will be calculated by multiplying the amount of their 2018 Bonus by their Elected Percentage times 1.2, divided by the closing price of the Company’s common stock on the date of grant. Under the Stock Bonus Election Program, an executive's eligibility to receive the 2018 Bonus Shares and Uplift Shares, if any, is contingent upon the executive’s continued employment with the Company through the date of grant.

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. The Company will reclassify the liability to equity at the time the shares are granted, if any.

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.

In May 2018, the Company granted its President and Chief Executive Officer, Franklin (Fritz) Hobbs (“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 20018 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, pending Mr. Hobbs' continued employment with the Company through December 31, 2020, the vesting date of the 2018 PSUs (collectively, the "2018 Shares Earned"). The unamortized expense for the 2018 Shares Earned will be recorded through the remainder of the service period. The grant of the 2018 PSUs is recorded in the PSU table below.

From 2015 through 2017, the Company granted PSUs with both market and service conditions to certain of its executives. 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 total shareholder return ("TSR") compared to 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.

The PSUs that included a market condition required the use of a Monte Carlo simulation approach to model future stock price movements based upon the risk-free rate of return, the date of return, the volatility of each entity and the pair-wise covariance between each entity. These results were 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.

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") 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 target, granted on March 31, 2017 and 7,750 shares, representing the 30% achievement over target, granted on March 31, 2018. The grant of the additional shares and the release of the earned shares, both of which occurred on March 31, 2018, are included in the PSU table below. 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 will be released to the three remaining executives holding such outstanding grants on March 31, 2019, subject to their continued employment with the Company through that date.

On April 1, 2016, the Company granted an aggregate of 131,250 PSUs with both market and service conditions to six of its executives (the "2016 PSUs"). In March 2018, the Compensation Committee determined that the performance metrics for the 2016 PSUs for the 2017 Performance Period had been achieved at the 130% level, and accordingly, 16,250 shares in the aggregate were released to the two remaining executives holding such outstanding grants, comprised of 12,500 shares, representing the 100% achievement target, granted on April 1, 2016 and 3,750 shares, representing the 30% achievement over target, granted on April 1, 2018. The grant of the additional shares and the release of the earned shares, both of which occurred on April 1, 2018, are included in the PSU table below. In February 2017, the Compensation Committee determined that the performance metrics for the 2016 PSUs for the 2016 Performance Period had been achieved at the 90.4% level, and accordingly, 24,106 shares in the aggregate were released to the four executives holding such outstanding grants on March 16, 2017. The unearned shares relating to the 2016 Performance Period, aggregating 2,560 shares, were forfeited on March 16, 2017. At December 31, 2018, there were no unvested 2016 PSUs outstanding.

On March 16, 2015, the Company granted an aggregate of 131,250 PSUs with both market and service conditions to eight of its executives (the "2015 PSUs"). In March 2018, the Compensation Committee determined that the performance metrics for the 2015 PSUs for the 2017 Performance Period had been achieved at the 112% level, and accordingly, 7,934 shares in the aggregate were released to the two remaining executives holding such outstanding grants, comprised of 7,084 shares, representing the 100% achievement target, granted on March 16, 2015 and 850 shares, representing the 12% achievement over target, granted on March 16, 2018. The grant of the additional shares and the release of the earned shares, both of which occurred on March 16, 2018, are included in the PSU table below. In February 2017, the Compensation Committee determined that the performance metrics for the 2015 PSUs for the 2016 Performance Period had been achieved at the 76.0% level, and accordingly, 23,750 shares in the aggregate were released to the four remaining executives holding such outstanding grants on April 1, 2017. The unearned shares relating to the 2016 Performance Period, aggregating 7,500 shares, were forfeited on April 1, 2017. At December 31, 2018, there were no unvested 2015 PSUs outstanding.

In connection with the Merger, as previously described above, PSUs held by the former CEO were converted to RSAs and certain other PSUs were accelerated and released in accordance with the individual employment terms of each PSU grantee. The vesting schedules of the remaining unvested PSUs were adjusted to continue to vest on their terms.

On December 14, 2017, the Company announced that, on and effective December 13, 2017, the Board of Directors appointed Franklin (Fritz) W. Hobbs as the Company's President and Chief Executive Officer, and the former CEO resigned his position as the Company's President and Chief Executive Officer and as a member of the Board of Directors. In connection with the separation of the former CEO from the Company and in accordance with his employment agreement with the Company, as amended, the Company accelerated the vesting of his then-unvested RSAs (including those that had been converted from PSUs in connection with the Merger, as described above). These shares are reported as "Vested" in the RSA table below. However, due to the terms of the former CEO's separation agreement with the Company and applicable employment laws, the shares were not released until January 2018. Accordingly, these shares were not considered outstanding as of December 31, 2017.

Stock Options

Options are issued to purchase shares of common stock of the Company at prices that are equal to the fair market value of the shares on the date the option is granted. Options granted under the Stock Plan expire ten years from the date of grant. Outstanding options under the Edgewater Plan expire ten years from the date of grant. Outstanding options under the 2008 Plan expire either seven or ten years from the date of grant. Outstanding options under the 2012 Plan expire ten years from the date of grant. The grant date fair value of 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, 2018 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, 2018	435,187		$	14.71
Granted	—		$	—
Edgewater outstanding options converted to Ribbon options	312,452		$	1.97
Exercised	(15,935	)	$	4.58
Forfeited	(35,431	)	$	1.81
Expired	(114,212	)	$	14.33
Outstanding at December 31, 2018	582,061		$	9.01	4.82	$	774
Vested or expected to vest at December 31, 2018	560,325		$	9.27	4.67	$	720
Exercisable at December 31, 2018	428,097		$	11.47	3.60	$	366

The Company did not grant options during the year ended December 31, 2018. The grant date fair values of options to purchase common stock granted in the years ended December 31, 2017 and 2016 were estimated using the Black-Scholes valuation model with the following assumptions:



	Year ended December 31,
	2017	2016
Risk-free interest rate	1.22% - 1.95%	1.00% - 1.61%
Expected dividends	—	—
Weighted average volatility	51.1%	54.8%
Expected life (years)	5.0	5.0-10.0

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 values of options granted were $3.05 for the year ended December 31, 2017 and $4.39 for the year ended December 31, 2016.

The total intrinsic values of options exercised were approximately $39,000 for the year ended December 31, 2018, $0.2 million for the year ended December 31, 2017 and approximately $42,000 for the year ended December 31, 2016.

The Company received cash from option exercises of $0.1 million in the year ended December 31, 2018, $0.6 million in the year ended December 31, 2017 and $0.2 million in the year ended December 31, 2016.

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, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	1,696,582		$	7.68
Granted	1,123,956		$	6.90
Vested	(1,035,952	)	$	8.05
Forfeited	(276,575	)	$	7.36
Unvested balance at December 31, 2018	1,508,011		$	6.90

The activity related to the Company's RSUs for the year ended December 31, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	17,932		$	6.90
Granted	908,300		$	6.27
Vested	(242,110	)	$	5.57
Forfeited	(47,822	)	$	6.76
Unvested balance at December 31, 2018	636,300		$	6.52

The total fair value of vested restricted stock grant shares was $9.7 million in the year ended December 31, 2018, $19.1 million in the year ended December 31, 2017 and $10.1 million in the year ended December 31, 2016.

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, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	60,834		$	9.65
Granted	207,350		$	5.78
Vested	(57,768	)	$	9.90
Forfeited	—		$	—
Unvested balance at December 31, 2018	210,416		$	5.77

The total fair value of vested performance-based stock grant shares was $0.6 million in the year ended December 31, 2018, $1.4 million in the year ended December 31, 2017 and $0.2 million in the year ended December 31, 2016.

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 of Directors 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 scheduled for May 31, 2019.

At December 31, 2018, 5.0 million shares, the maximum number of shares that may be issued under the ESPP, were authorized, and 1.4 million shares were available under the ESPP for future issuance.

Stock-Based Compensation

The consolidated statements of operations included stock-based compensation for the years ended December 31, 2018, 2017 and 2016 as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Product cost of revenue	$	114	$	514	$	359
Service cost of revenue	345		1,448		1,314
Research and development	1,797		7,337		5,014
Sales and marketing	2,935		4,885		6,209
General and administrative	5,881		11,473		6,872
	$	11,072	$	25,657	$	19,768

There was no income tax benefit for employee stock-based compensation expense for the years ended December 31, 2018, 2017 and 2016 due to the valuation allowance recorded.

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 the former CEO in connection with his separation from the Company effective December 13, 2017. These incremental amounts were all recorded in the fourth quarter of 2017.

During the three months ended March 31, 2017, the Company reversed $1.0 million of incremental expense to correct an error in 2016 related to the acceleration of certain stock awards held by an executive who separated from the Company in 2016. Management had reviewed and considered the impact of the error and determined that it was not material to the Company's consolidated financial results for the third and fourth quarters of 2016, as well as the 2016 fiscal year. Management has also determined that the correction of this error is not material to the results of operations for the 2017 reporting periods.

At December 31, 2018, there was $10.7 million, net of expected forfeitures, of unrecognized stock-based compensation expense related to unvested stock options, RSAs, RSUs, PSUs and the ESPP. 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, 2018 consists of the following:



Amended and Restated Stock Incentive Plan	3,501,214
ESPP	1,431,513
	4,932,727

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.

EMPLOYEE DEFINED CONTRIBUTION PLANS

12 Months Ended

Dec. 31, 2018

Retirement Benefits [Abstract]

EMPLOYEE DEFINED CONTRIBUTION PLAN

EMPLOYEE DEFINED CONTRIBUTION PLANS

The Company offers 401(k) savings plans to eligible employees. In June 2016, at the recommendation of the Compensation Committee, the Company’s Board of Directors elected to reinstate a discretionary limited 401(k) match program of up to $2,000 per year ($1,000 per each half-year) per eligible employee, contingent upon the Company’s achievement of certain financial metric targets set by the Compensation Committee. The matching contribution became effective July 1, 2016.

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 $3.2 million in the year ended December 31, 2018, $1.4 million in the year ended December 31, 2017 and $0.6 million in the year ended December 31, 2016.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

In connection with the Merger, the Company assumed GENBAND's defined benefit retirement plans that cover some 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, 2018, in conjunction with the Merger Restructuring Initiative, there were reductions in force that significantly reduced benefits that can be earned under the 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 year ended December 31, 2018 and for the period from the Merger Date to December 31, 2017, the funded status of the plans and the amounts recognized in the consolidated balance sheets as of December 31, 2018 and 2017 were as follows (in thousands):



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Changes in projected benefit obligations:
Projected benefit obligation, beginning of period	$	11,484		$	10,515
Service cost	449			68
Interest cost	150			25
Participant contributions	5			5
Benefits paid	(23		)	(3		)
Net actuarial loss on obligation	(414		)	562
Curtailment	(553		)	—
Settlement	(250		)	—
Currency loss	—			312
Projected benefit obligation, end of year	$	10,848		$	11,484

Changes in plan assets:
Fair value of plan assets, beginning of period	$	3,893		$	3,776
Actual return on plan assets	(53		)	(8		)
Employer contributions	292			22
Participant contributions	5			5
Administrative expenses	(22		)	(4		)
Benefits paid	(273		)	(3		)
Currency gain	—			105
Fair value of plan assets, end of year	$	3,842		$	3,893

Funded status at end of year	$	(7,006	)	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	222		$	578

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(75	)	$	(67	)
Other long-term liabilities (non-current pension liability)	(6,931		)	(7,524		)
Net amount recognized	$	(7,006	)	$	(7,591	)

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2018 and 2017 were as follows (in thousands):



	December 31, 2018		December 31, 2017
Aggregate projected benefit obligation	$	10,848	$	11,484
Aggregate accumulated benefit obligation	$	7,152	$	7,793
Aggregate fair value of plan assets	$	3,842	$	3,893

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



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	449		$	68
Interest cost	150			25
Expected return on plan assets	(45		)	(8		)
Plan asset expenses	22			4
Curtailment gain	(510		)	—
Settlement loss	3			—
Net periodic benefit costs	$	69		$	89

The Company made benefit payments of $273,000 in the year ended December 31, 2018, including $250,000 of one-time lump sum payments to participants, and $3,000 in the period from the Merger Date to December 31, 2017. Expected benefit payments for the next ten years are as follows:



Years ending December 31,
2019	$	75
2020	45
2021	94
2022	47
2023	231
2024 to 2028	643
	$	1,135

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



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Net (gain) loss	$	(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 $4,000 for the year ended December 31, 2019.

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2018 and 2017 were as follows:



	December 31, 2018		December 31, 2017
Discount rate	1.30	%	1.31	%
Rate of compensation increase	2.83	%	3.38	%

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



	Year ended December 31, 2018		October 27, 2017 to December 31, 2017
Discount rate	1.50	%	1.49	%
Expected long-term return on plan assets	1.34	%	1.23	%
Rate of compensation increase	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 $3.8 million at December 31, 2018 and $3.9 million at December 31, 2017. 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 year ended December 31, 2018, employees in the Netherlands and Switzerland made contributions to the respective pension plans aggregating $5,000. 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 year ended December 31, 2018, the Company contributed $292,000 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 2019.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS

12 Months Ended

Dec. 31, 2018

Retirement Benefits [Abstract]

Pension Disclosure

EMPLOYEE DEFINED CONTRIBUTION PLANS

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.

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Changes in projected benefit obligations:
Projected benefit obligation, beginning of period	$	11,484		$	10,515
Service cost	449			68
Interest cost	150			25
Participant contributions	5			5
Benefits paid	(23		)	(3		)
Net actuarial loss on obligation	(414		)	562
Curtailment	(553		)	—
Settlement	(250		)	—
Currency loss	—			312
Projected benefit obligation, end of year	$	10,848		$	11,484

Changes in plan assets:
Fair value of plan assets, beginning of period	$	3,893		$	3,776
Actual return on plan assets	(53		)	(8		)
Employer contributions	292			22
Participant contributions	5			5
Administrative expenses	(22		)	(4		)
Benefits paid	(273		)	(3		)
Currency gain	—			105
Fair value of plan assets, end of year	$	3,842		$	3,893

Funded status at end of year	$	(7,006	)	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	222		$	578

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(75	)	$	(67	)
Other long-term liabilities (non-current pension liability)	(6,931		)	(7,524		)
Net amount recognized	$	(7,006	)	$	(7,591	)

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2018 and 2017 were as follows (in thousands):



	December 31, 2018		December 31, 2017
Aggregate projected benefit obligation	$	10,848	$	11,484
Aggregate accumulated benefit obligation	$	7,152	$	7,793
Aggregate fair value of plan assets	$	3,842	$	3,893

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



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	449		$	68
Interest cost	150			25
Expected return on plan assets	(45		)	(8		)
Plan asset expenses	22			4
Curtailment gain	(510		)	—
Settlement loss	3			—
Net periodic benefit costs	$	69		$	89



Years ending December 31,
2019	$	75
2020	45
2021	94
2022	47
2023	231
2024 to 2028	643
	$	1,135



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

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2018 and 2017 were as follows:



	December 31, 2018		December 31, 2017
Discount rate	1.30	%	1.31	%
Rate of compensation increase	2.83	%	3.38	%

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



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

INCOME TAXES

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Income (loss) before income taxes:
United States	$	(52,569	)	$	(55,932	)	$	(11,973	)
Foreign	(20,841		)	2,240			557
	$	(73,410	)	$	(53,692	)	$	(11,416	)

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



	Year ended December 31,
	2018			2017			2016
Provision (benefit) for income taxes:
Current:
Federal	$	561		$	(200	)	$	12
State	128			115			24
Foreign	2,198			1,960			1,378
Total current	2,887			1,875			1,414
Deferred:
Federal	(8,481		)	49,570			(301		)
State	(1,414		)	(4,833		)	(1,007		)
Foreign	(1,477		)	(816		)	338
Change in valuation allowance	11,885			(64,236		)	2,072
Total deferred	513			(20,315		)	1,102
Total	$	3,400		$	(18,440	)	$	2,516

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



	Year ended December 31,
	2018		2017		2016
U.S. statutory income tax rate	21.0	%	35.0	%	35.0	%
State income taxes, net of federal benefit	(0.1	)	1.2		—
Foreign income taxes	(5.7	)	(0.5	)	(7.9	)
Foreign tax audit	—		—		(5.2	)
Acquisition costs	(0.3	)	(6.0	)	—
Foreign deemed dividends	(3.4	)	(3.8	)	(5.0	)
Stock-based compensation	(0.3	)	26.8		(38.9	)
Tax credits	0.6		(33.3	)	11.6
Uncertain tax positions	1.3		(1.2	)	—
NOL and credit limitations	—		(18.9	)	—
Valuation allowance	(16.1	)	29.0		(1.9	)
Goodwill amortization	0.3		(1.7	)	(6.7	)
Meals and entertainment	(0.4	)	(0.5	)	(1.4	)
Tax reform	—		8.8		—
Other, net	(1.5	)	(0.6	)	(1.6	)
Effective income tax rate	(4.6	)%	34.3	%	(22.0	)%

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



	December 31,
	2018			2017
Assets:
Net operating loss carryforwards	$	76,278		$	58,624
Research and development tax credits	28,664			24,499
Deferred revenue	5,755			5,886
Accrued expenses	9,601			10,786
Inventory	4,906			5,980
Stock-based compensation	1,536			727
Fixed assets	7,716			7,452
Other temporary differences	1,943			1,556
	136,399			115,510
Valuation allowance	(101,484		)	(89,600		)
Total deferred tax assets	34,915			25,910
Liabilities:
Purchased intangible assets	(26,014		)	(17,092		)
Unremitted foreign income	(4,487		)	(3,171		)
Total deferred tax liabilities	(30,501		)	(20,263		)
Total net deferred tax assets	$	4,414		$	5,647

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	$	9,152		$	8,434
Deferred income taxes - noncurrent liabilities	(4,738		)	(2,787		)
	$	4,414		$	5,647

At December 31, 2018, the Company had cumulative federal and state net operating losses ("NOLs") of $289.1 million. The federal NOL carryforwards expire at various dates from 2020 through 2037. The state NOL carryforwards expire at various dates from 2018 through 2038.

The Company also has available federal, state and foreign income tax credit carryforwards of $28.7 million that expire at various dates from 2019 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, 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. 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.

During 2018 and 2017, 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 $82.4 million at December 31, 2018 and $73.1 million at December 31, 2017. 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 $9.5 million at December 31, 2018 and $6.2 million at December 31, 2017. In analyzing the deferred tax assets related to the Company's Canada subsidiaries, 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 $9.6 million. The deferred tax assets recognized with no valuation allowance at December 31, 2018 and 2017 relate to foreign subsidiaries where recoverability is concluded to be more likely than not based on the Company's cost plus compensation policy.

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



	2018			2017			2016
Unrecognized tax benefits at January 1	$	4,528		$	8,969		$	8,888
Increases related to current year tax positions	74			139			36
Increases related to prior period tax positions	122			430			723
Increases related to business acquisitions	—			2,012			—
Decreases related to prior period tax positions	(1,263		)	(7,022		)	(81		)
Settlements	—			—			(597		)
Unrecognized tax benefits at December 31	$	3,461		$	4,528		$	8,969

The Company recorded liabilities for potential penalties and interest of $0.1 million for the year ended December 31, 2018, $0.2 million for the year ended December 31, 2017 and $0.1 million for the year ended December 31, 2016. The Company had cumulative deferred tax liabilities recorded related to interest and penalties of $0.6 million for the year ended December 31, 2018, $0.6 million for the year ended December 31, 2017 and $0.2 million for the year ended December 31, 2016. Some of the unrecognized tax benefit items are expected to reverse in 2019 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 2015 through 2018 remain open to examination by the major taxing jurisdictions to which the Company is subject. The Company's federal NOLs generated prior to 2015 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, 2018, 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 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 will be filing 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.

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 will be filing 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.

The 2016 acquisition of Taqua was a taxable purchase of a business under Section 197 of the Internal Revenue Code. The tax amortization related to Taqua goodwill created a deferred tax liability.

RELATED PARTIES	12 Months Ended
RELATED PARTIES	Dec. 31, 2018
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 does not amortize and the principal thereon is payable in full on the third anniversary of its execution. Interest on the promissory note is 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, constitutes an event of default under the promissory note. If an event of default occurs under the promissory note, the payees may 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. 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.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

Leases

The Company leases its facilities under operating leases, which expire at various times through 2029. The Company is responsible for certain real estate taxes, utilities and maintenance costs under these leases. The Company's corporate headquarters is located in a leased facility in Westford, Massachusetts, consisting of 97,500 square feet under a lease that expires in August 2028.

Escalation clauses, free rent and other lease concessions are recognized on a straight-line basis over the minimum lease term. Rent expense was $11.9 million for the year ended December 31, 2018, $5.9 million for the year ended December 31, 2017 and $4.5 million for the year ended December 31, 2016.

Future minimum payments under operating lease arrangements as of December 31, 2018 were as follows (in thousands):



Years ending December 31,
2019	$	10,705
2020	8,384
2021	7,455
2022	5,691
2023	5,430
Thereafter	19,818
	$	57,483

Litigation and Contingencies

The Company fully cooperated with an SEC inquiry regarding the development and issuance of Sonus’ first quarter 2015 revenue and earnings guidance. The Company and the SEC reached an agreement to resolve this matter and on August 7, 2018, the SEC issued a Cease and Desist Order (the "SEC Cease and Desist Order"). As part of the Order, the findings of which the Company neither admitted to nor denied, the Company agreed to pay a $1.9 million civil penalty and agreed not to violate the securities laws in the future. The Company recorded $1.9 million in the year ended December 31, 2017 for then-potential fines in connection with this investigation and has paid such amount to the SEC.

The Company is involved in six lawsuits with Metaswitch Networks Ltd., Metaswitch Networks Corp. and Metaswitch Inc. (collectively, “Metaswitch”). In five of the lawsuits, the Company is the plaintiff and, in three of those five lawsuits, the Company is also a counterclaim defendant. In the sixth case, the Company is the defendant. On January 21, 2014, GENBAND and the Company’s indirectly-owned subsidiary, GENBAND US LLC, filed a complaint in the Eastern District of Texas, Marshall Division, alleging that Metaswitch infringed certain patents owned by GENBAND. Following unsuccessful mediation, a trial took place and on January 15, 2016, the jury awarded $8.2 million in past royalty damages to GENBAND, which neither GENBAND nor the Company has recorded. On September 29, 2016, the court confirmed the jury verdict following motions from both parties. On March 22, 2018, the district court entered final judgment awarding GENBAND $8.9 million in royalties for damages through January 15, 2016 at rates set by the district court, excluding pre- and post-judgment interest and costs. On April 10, 2018, the clerk of the court set the awarded costs at $0.4 million. On April 19, 2018, Metaswitch filed a notice of appeal on the judgment with United States Court of Appeals for the Federal Circuit, and filed its appeal brief on July 6, 2018. Oral argument on the appeal is set for March 8, 2019.

On April 18, 2018, through Sonus, the Company filed a complaint in the Eastern District of Texas, Marshall Division, alleging that Metaswitch is continuing to infringe the patents from the first lawsuit above through sales of Metaswitch's allegedly "redesigned" products. This suit seeks a finding that Metaswitch's infringement is willful. This suit also alleges false advertising and seeks monetary damages resulting from allegedly false and misleading statements Metaswitch made regarding the first lawsuit. The district court has set trial for September 9, 2019.

Through Sonus and GENBAND US LLC, the Company is involved as plaintiff and counterclaim defendant in a lawsuit with Metaswitch regarding claims that Metaswitch misappropriated trade secrets of GENBAND, and the Company is seeking monetary damages. This case is pending in state court in Dallas County, Texas, and stems from claims originally brought in a patent lawsuit between GENBAND and Metaswitch. The state court action was filed on March 28, 2017. Metaswitch filed its answer on April 21, 2017, in which it asserted counterclaims against GENBAND. On July 11, 2018, Metaswitch filed its fifth amended answer and counterclaims against GENBAND. The Texas state court has set a special setting for a trial for this case on April 22, 2019.

Through Sonus, the Company is also involved as plaintiff and counterclaim defendant in two patent infringement lawsuits with Metaswitch asserting the infringement of a total of ten patents that came into the Company from Sonus, and the Company is seeking monetary damages. Sonus filed these two lawsuits in the Eastern District of Texas, Marshall Division, on March 8, 2018. Metaswitch filed its answers on May 15, 2018, in which it asserted counterclaims against Sonus, including alleged infringement by the Company and Sonus of a total of ten patents. The district court has set trials for these cases to occur on February 18, 2020 and June 15, 2020.

On November 19, 2018, Metaswitch filed a complaint against the Company and several of its subsidiaries in the Southern District of New York, alleging various antitrust violations based, in large part, on allegations that GENBAND should not have brought its successful patent infringement lawsuit against Metaswitch. Metaswitch is seeking monetary damages. The Company has not yet filed an answer, and the court has not yet set a schedule.

At this time, it is not possible to predict the outcome of the litigation matters with Metaswitch, but the Company does not expect the results of any of these actions to have a material adverse effect on the Company's business or consolidated financial results.

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, Raymond P. Dolan, Mark T. Greenquist and Michael Swade (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 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 the SEC 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. Briefing on the issue was completed on January 30, 2019 and the Massachusetts District Court is expected to issue a decision shortly. The Company has not yet filed an answer, and the Massachusetts District Court has not yet set a schedule.

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, 2018

Quarterly Financial Information Disclosure [Abstract]

QUARTERLY RESULTS (UNAUDITED)

The following tables present the Company's quarterly operating results for the years ended December 31, 2018 and 2017. 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			Second Quarter			Third Quarter (1)			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



	First Quarter			Second Quarter			Third Quarter		Fourth Quarter (2)
	(In thousands, except per share data)
Fiscal 2017
Revenue	$	53,368		$	55,733		$	74,629	$	146,212
Cost of revenue	19,620			19,331			20,082		69,413
Gross profit	$	33,748		$	36,402		$	54,547	$	76,799
Loss (income) from operations	$	(10,782	)	$	(12,703	)	$	3,919	$	(35,663	)
Net (loss) income	$	(10,646	)	$	(12,345	)	$	3,453	$	(15,714	)
(Loss) earnings per share (3):
Basic	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Diluted	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Shares used in computing (loss) diluted earnings per share:
Basic	49,114			49,543			49,753		86,567
Diluted	49,114			49,543			50,131		86,567

__________________________________


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


(2)	Includes the results of GENBAND for the period subsequent to October 27, 2017.


(3)	Income (loss) per share is calculated independently for each of the quarters presented; accordingly, the sum of the quarterly earnings (loss) 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, 2018
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 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 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.
Reclassifications	Reclassifications Certain reclassifications, not 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 generally 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 vs the total estimated labor time required to complete the performance obligation. Revenue is recorded proportionally as costs are incurred or 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 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 remaining 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.
Restricted Cash	Restricted Cash The Company classifies as restricted cash all cash pledged as collateral to secure long-term obligations and all cash whose use is otherwise limited by contractual provisions. Restricted cash is recorded within other current assets on the consolidated balance sheet.
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. Realized and unrealized foreign currency 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 comprised of certain intangible assets arising from the Merger, the Edgewater Acquisition and previous acquisitions. 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. The Company performed its step one assessments for each of the years ended December 31, 2018, 2017 and 2016 and determined each year that its fair value was in excess of its carrying value and accordingly, there was no impairment of goodwill.
Share-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.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred.
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. Advertising expenses were $0.5 million for the year ended December 31, 2018, $0.3 million for the year ended December 31, 2017 and $0.1 million for the year ended December 31, 2016.
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 maker is its President and Chief Executive Officer.
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. In certain cases, Ribbon has been 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 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, 2018, 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 $4.5 million, primarily consisting of withholding and distribution taxes, relating to undistributed earnings from these subsidiaries as of December 31, 2018. 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 In May 2017, the Financial Accounting Standards Board ("FASB") issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"), which amends the scope of modification accounting for share-based payment arrangements such that an entity would not apply modification accounting if the fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. ASU 2017-09 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The adoption of ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In March 2017, the FASB issued ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ("ASU 2017-07"). ASU 2017-07 amends the requirements in ASC 715, Compensation - Retirement Benefits ("ASC 715") to require entities to disaggregate the current-service-cost component from the other components of net benefit cost (the "other components") and include it with other current compensation costs for related employees, present the other components elsewhere in the income statement and outside of income from operations if such a subtotal is presented and disclose the income statement lines that contain the other components if they are not presented on appropriately described separate lines. ASU 2017-07 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. The adoption of ASU 2017-07 did not have a material impact on the Company's consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), which adds or clarifies guidance on eight cash flow issues, including debt prepayment or debt extinguishment costs, settlement of zero-coupon debt instruments or certain other debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, proceeds from the settlement of corporate-owned life insurance policies, distributions received from equity method investees, beneficial interests in securitization transactions and separately identifiable cash flows and application of the predominance principle. ASU 2016-15 became effective for the Company beginning January 1, 2018 for both interim and annual reporting periods. Entities must apply the guidance retrospectively to all periods presented but may apply it prospectively from the earliest date practicable if retrospective application would be impracticable. The adoption of ASU 2016-15 did not have a material impact on the Company's 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 is effective for the Company beginning January 1, 2020. The Company is currently assessing the potential impact of the adoption of ASU 2018-15 on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Section A - Leases: Amendments to the FASB Accounting Standards Codification ("ASU 2016-02"), its new standard on accounting for leases. ASU 2016-02 introduces a lessee model that brings most leases onto the balance sheet. ASU 2016-02 eliminates the current GAAP requirement for an entity to use bright-line tests in determining lease classification. ASU 2016-02 became effective for the Company for both interim and annual periods beginning January 1, 2019. Upon adoption of ASU 2016-02, the Company will recognize lease obligations for the right to use these assets in connection with its existing lease agreements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements ("ASU 2018-11") and ASU 2018-10, Codification Improvements to Topic 842, Leases, both of which provided improvements to certain aspects of the guidance in ASC 842, Leases. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842, which provided additional clarification and implementation guidance. The Company has elected to use the alternative transition method as described in ASU 2018-11, which allows entities to initially apply ASU 2016-02 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings, with no subsequent adjustments to prior period lease costs for comparability. To illustrate the magnitude of this change, the amount of the Company's off-balance sheet operating leases at December 31, 2018 is disclosed in Note 22. Beginning on January 1, 2019, the Company's operating leases, excluding those with terms less than 12 months, were discounted and recorded as assets and liabilities in the Company's consolidated balance sheet. 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 2018-14, 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 to add, remove and clarify disclosure requirements related to defined benefit pension and other postretirement plans. ASU 2018-14 is 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 (“ASC 820”). ASU 2018-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting. In July 2018, the FASB issued ASU 2018-09, Codification Improvements (“ASU 2018-09”), which contains amendments to clarify, correct errors in or make minor improvements to the 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. ASU 2018-09 became effective for the Company beginning January 1, 2019. In June 2018, the FASB issued ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"), 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. ASU 2018-07 became effective for the Company beginning January 1, 2019. 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 ("ASU 2018-02"), 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. ASU 2018-02 became effective for the Company beginning January 1, 2019. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ("ASU 2016-16"), 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. ASU 2016-16 became effective for the Company beginning January 1, 2019 for both interim and annual reporting periods. 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. ASU 2016-13 is effective for the Company beginning January 1, 2020 for both interim and annual reporting periods, with early adoption permitted.

BUSINESS ACQUISITONS (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018, 2017 and 2016 are were as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Professional and services fees (acquisition-related)	$	7,627	$	11,916	$	1,152
Management bonuses (acquisition-related)	1,972		931		—
Integration-related expenses	7,352		1,916		—
	$	16,951	$	14,763	$	1,152

Edgewater

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

A summary of the preliminary 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]

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			2016
	(unaudited)
Revenue	$	615,286		$	631,914
Net loss	$	(69,741	)	$	(147,394	)
Loss per share	$	(0.69	)	$	(1.46	)

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

Taqua, LLC

Business Acquisition [Line Items]

Schedule of Recognized Identified Assets Acquired and Liabilities Assumed

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



Fair value of consideration transferred:
Cash, net of cash acquired	$	19,919

Fair value of assets acquired and liabilities assumed:
Current assets	$	3,347
Property and equipment	1,478
Intangible assets:
Developed technology	2,100
Customer relationships	9,510
Goodwill	9,581
Other noncurrent assets	23
Current liabilities	(5,435		)
Long-term liabilities	(685		)
	$	19,919

CASH EQUIVALENTS AND INVESTMENTS (Tables)

12 Months Ended

Dec. 31, 2018

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 investments at December 31, 2018 and 2017 were comprised of the following (in thousands):



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

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



	December 31, 2017
	Amortized cost		Unrealized gains		Unrealized losses			Fair value
Cash equivalents	$	1,254	$	—	$	—		$	1,254

Short-term investments
U.S. government agency notes	$	4,091	$	—	$	(19	)	$	4,072
Corporate debt securities	8,048		—		(31		)	8,017
Certificates of deposit	5,135		—		—			5,135
	$	17,274	$	—	$	(50	)	$	17,224
Investments
U.S. government agency notes	$	3,992	$	—	$	(28	)	$	3,964
Corporate debt securities	3,908		—		(24		)	3,884
Certificates of deposit	1,183		—		—			1,183
	$	9,083	$	—	$	(52	)	$	9,031

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	$	—	$	—

Short-term investments
U.S. government agency notes	$	3,989	$	—	$	3,989	$	—
Corporate debt securities	3,295		—		3,295		—
	$	7,284	$	—	$	7,284	$	—



			Fair value measurements at December 31, 2017 using:
	Total carrying value at December 31, 2017		Quoted prices in active markets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
Cash equivalents	$	1,254	$	1,254	$	—	$	—

Short-term investments
U.S. government agency notes	$	4,072	$	—	$	4,072	$	—
Corporate debt securities	8,017		—		8,017		—
Certificates of deposit	5,135		—		5,135		—
	$	17,224	$	—	$	17,224	$	—
Investments
U.S. government agency notes	$	3,964	$	—	$	3,964	$	—
Corporate debt securities	3,884		—		3,884		—
Certificates of deposit	1,183		—		1,183		—
	$	9,031	$	—	$	9,031	$	—

ACCOUNTS RECEIVABLE, NET (Tables)

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Schedule of accounts receivable, net

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



	December 31,
	2018			2017
Accounts receivable	$	188,522		$	165,229
Allowance for doubtful accounts	(669		)	(73		)
Accounts receivable, net	$	187,853		$	165,156

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
2018	$	73	$	351	$	620		$	(375	)	$	669
2017	$	10	$	154	$	(56	)	$	(35	)	$	73
2016	$	10	$	10	$	—		$	(10	)	$	10

INVENTORY (Tables)

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

Schedule of inventory

Inventory consisted of the following (in thousands):



	December 31,
	2018			2017
On-hand final assemblies and finished goods inventories	$	19,879		$	18,374
Deferred cost of goods sold	3,798			4,569
	23,677			22,943
Less noncurrent portion (included in Other assets)	(1,075		)	(1,640		)
Current portion	$	22,602		$	21,303

PROPERTY AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Schedule of property and equipment

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



		December 31,
	Useful Life	2018			2017
Equipment	2-5 years	$	76,423		$	67,415
Software	2-5 years	24,707			21,977
Furniture and fixtures	3-5 years	1,490			1,892
Leasehold improvements	Shorter of the life of the lease or estimated useful life (1-5 years)	21,220			18,428
		123,840			109,712
Less accumulated depreciation and amortization		(96,798		)	(84,932		)
Property and equipment, net		$	27,042		$	24,780

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



	December 31,
	2018			2017
Cost	$	2,979		$	664
Less accumulated depreciation	(875		)	(180		)
Property and equipment under capital leases, net	$	2,104		$	484

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



	December 31,
	2018		2017
United States	$	17,862	$	17,576
Canada	4,076		1,740
Asia/Pacific	3,841		3,853
Europe	1,100		1,400
Other	163		211
	$	27,042	$	24,780

INTANGIBLE ASSETS AND GOODWILL (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of intangible assets

The Company's intangible assets at December 31, 2018 and 2017 consisted of the following (in thousands):



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



December 31, 2017	Weighted average amortization period (years)	Cost		Accumulated amortization		Net carrying value
In-process research and development	*	$	5,600	$	—	$	5,600
Developed technology	6.90	153,380		24,211		129,169
Customer relationships	9.32	120,840		12,015		108,825
Trade names	3.00	900		80		820
Internal use software	3.00	730		730		—
	7.77	$	281,450	$	37,036	$	244,414

* 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, 2018, 2017 and 2016 was as follows (in thousands):



	Year ended December 31,						Statement of operations classification
	2018		2017		2016		Statement of operations classification
Developed technology	$	38,976	$	18,358	$	6,038	Cost of revenue - product
Customer relationships	10,203		4,145		1,462		Sales and marketing
Trade names	544		80		—		Sales and marketing
	$	49,723	$	22,583	$	7,500

Schedule of estimated future amortization expense for intangible assets

Estimated future amortization expense for the Company's intangible assets at December 31, 2018 was as follows (in thousands):



Years ending December 31,
2019	$	47,411
2020	46,552
2021	40,571
2022	34,156
2023	26,756
Thereafter	55,945
	$	251,391

Schedule of goodwill

The changes in the carrying value of the Company's goodwill in the years ended December 31, 2018 and 2017 were as follows (in thousands):



	Year ended December 31,
	2018			2017
Balance at January 1
Goodwill	$	338,822		$	52,499
Accumulated impairment losses	(3,106		)	(3,106		)
	335,716			49,393
Acquisition of Edgewater	48,053			—
Acquisition of GENBAND	—			285,825
Acquisition of Taqua and subsequent purchase accounting adjustments	—			498
Write-off of goodwill attributable to dissolved subsidiary	(114		)	—
Balance at December 31	$	383,655		$	335,716

RESTRUCTURING ACCRUALS (Tables)

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

Summary of restructuring accrual activity

Summaries of the Taqua Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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
Facilities	$	365	$	—	$	—	$	(332	)	$	33



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	384	$	245	$	(49	)	$	(580	)	$	—
Facilities	218		508		—			(361		)	365
	$	602	$	753	$	(49	)	$	(941	)	$	365

Summaries of the Merger Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate		Cash payments			Balance at December 31, 2017
Severance	$	—	$	8,508	$	—	$	(913	)	$	7,595

Summaries of the 2016 Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



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
Facilities	$	95	$	—	$	—	$	(37	)	$	58



Year ended December 31, 2017	Balance at January 1, 2017		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	497	$	405	$	(26	)	$	(876	)	$	—
Facilities	—		126		—			(31		)	95
	$	497	$	531	$	(26	)	$	(907	)	$	95

Summaries of the GENBAND Restructuring Initiative accrual activity for the years ended December 31, 2018 and 2017 are as follows (in thousands):



Year ended December 31, 2018	Balance at January 1, 2018		Liability assumed in connection with Merger		Initiatives charged to expense		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



Year ended December 31, 2017	Balance at January 1, 2017		Liability assumed in connection with Merger		Initiatives charged to expense		Adjustments for changes in estimate			Cash payments			Balance at December 31, 2017
Severance	$	—	$	3,663	$	—	$	(158	)	$	(1,589	)	$	1,916
Facilities	—		431		—		(123		)	(103		)	205
	$	—	$	4,094	$	—	$	(281	)	$	(1,692	)	$	2,121

REVENUE RECOGNITION (Tables)

12 Months Ended

Dec. 31, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue

The Company's total revenue for the years ended December 31, 2018, 2017 and 2016 was disaggregated geographically as follows:



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, 2016	Product revenue		Service revenue (maintenance)		Service revenue (professional services)		Total revenue
United States	$	100,589	$	60,760	$	12,630	$	173,979
Europe, Middle East and Africa	21,073		10,769		2,021		33,863
Japan	12,258		9,831		3,524		25,613
Other Asia Pacific	8,226		3,478		827		12,531
Other	4,235		2,157		213		6,605
	$	146,381	$	86,995	$	19,215	$	252,591



	Year ended December 31,
	2018		2017		2016
Indirect sales through channel program	$	69,232	$	43,138	$	38,204
Direct sales	209,782		137,981		108,177
	$	279,014	$	181,119	$	146,381

The Company's product revenue from sales to enterprise customers and from sales to service provider customers for the years ended December 31, 2018, 2017 and 2016 was as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Sales to enterprise customers	$	57,534	$	35,592	$	28,508
Sales to service provider customers	221,480		145,527		117,873
	$	279,014	$	181,119	$	146,381

Schedule of Customer Assets and Liabilities

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



	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

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

STOCK-BASED COMPENSATION PLANS (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [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, 2018 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, 2018	435,187		$	14.71
Granted	—		$	—
Edgewater outstanding options converted to Ribbon options	312,452		$	1.97
Exercised	(15,935	)	$	4.58
Forfeited	(35,431	)	$	1.81
Expired	(114,212	)	$	14.33
Outstanding at December 31, 2018	582,061		$	9.01	4.82	$	774
Vested or expected to vest at December 31, 2018	560,325		$	9.27	4.67	$	720
Exercisable at December 31, 2018	428,097		$	11.47	3.60	$	366

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 options to purchase common stock granted in the years ended December 31, 2017 and 2016 were estimated using the Black-Scholes valuation model with the following assumptions:



	Year ended December 31,
	2017	2016
Risk-free interest rate	1.22% - 1.95%	1.00% - 1.61%
Expected dividends	—	—
Weighted average volatility	51.1%	54.8%
Expected life (years)	5.0	5.0-10.0

Schedule of activity related to unvested restricted stock grants

The activity related to the Company's RSAs for the year ended December 31, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	1,696,582		$	7.68
Granted	1,123,956		$	6.90
Vested	(1,035,952	)	$	8.05
Forfeited	(276,575	)	$	7.36
Unvested balance at December 31, 2018	1,508,011		$	6.90

The activity related to the Company's RSUs for the year ended December 31, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	17,932		$	6.90
Granted	908,300		$	6.27
Vested	(242,110	)	$	5.57
Forfeited	(47,822	)	$	6.76
Unvested balance at December 31, 2018	636,300		$	6.52

Schedule of activity related to performance stock awards

The activity related to the Company's PSUs for the year ended December 31, 2018 was as follows:



	Shares		Weighted Average Grant Date Fair Value
Unvested balance at January 1, 2018	60,834		$	9.65
Granted	207,350		$	5.78
Vested	(57,768	)	$	9.90
Forfeited	—		$	—
Unvested balance at December 31, 2018	210,416		$	5.77

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, 2018, 2017 and 2016 as follows (in thousands):



	Year ended December 31,
	2018		2017		2016
Product cost of revenue	$	114	$	514	$	359
Service cost of revenue	345		1,448		1,314
Research and development	1,797		7,337		5,014
Sales and marketing	2,935		4,885		6,209
General and administrative	5,881		11,473		6,872
	$	11,072	$	25,657	$	19,768

Schedule of common stock reserved for future issuance

Common stock reserved for future issuance at December 31, 2018 consists of the following:



Amended and Restated Stock Incentive Plan	3,501,214
ESPP	1,431,513
	4,932,727

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

12 Months Ended

Dec. 31, 2018

Retirement Benefits [Abstract]

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



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Changes in projected benefit obligations:
Projected benefit obligation, beginning of period	$	11,484		$	10,515
Service cost	449			68
Interest cost	150			25
Participant contributions	5			5
Benefits paid	(23		)	(3		)
Net actuarial loss on obligation	(414		)	562
Curtailment	(553		)	—
Settlement	(250		)	—
Currency loss	—			312
Projected benefit obligation, end of year	$	10,848		$	11,484

Changes in plan assets:
Fair value of plan assets, beginning of period	$	3,893		$	3,776
Actual return on plan assets	(53		)	(8		)
Employer contributions	292			22
Participant contributions	5			5
Administrative expenses	(22		)	(4		)
Benefits paid	(273		)	(3		)
Currency gain	—			105
Fair value of plan assets, end of year	$	3,842		$	3,893

Funded status at end of year	$	(7,006	)	$	(7,591	)

Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	$	222		$	578

Amounts recognized in the consolidated balance sheets consist of:
Accrued expenses and other (current pension liability)	$	(75	)	$	(67	)
Other long-term liabilities (non-current pension liability)	(6,931		)	(7,524		)
Net amount recognized	$	(7,006	)	$	(7,591	)

Schedule of Accumulated Benefit Obligations

Plans with underfunded or non-funded accumulated benefit obligations at December 31, 2018 and 2017 were as follows (in thousands):



	December 31, 2018		December 31, 2017
Aggregate projected benefit obligation	$	10,848	$	11,484
Aggregate accumulated benefit obligation	$	7,152	$	7,793
Aggregate fair value of plan assets	$	3,842	$	3,893

Schedule of Net Benefit Costs

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



	Year ended December 31, 2018			October 27, 2017 to December 31, 2017
Service cost	$	449		$	68
Interest cost	150			25
Expected return on plan assets	(45		)	(8		)
Plan asset expenses	22			4
Curtailment gain	(510		)	—
Settlement loss	3			—
Net periodic benefit costs	$	69		$	89

Schedule of Expected Benefit Payments

Expected benefit payments for the next ten years are as follows:



Years ending December 31,
2019	$	75
2020	45
2021	94
2022	47
2023	231
2024 to 2028	643
	$	1,135

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



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

Schedule of Assumptions Used

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



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

The principal weighted average assumptions used to determine the benefit obligation at December 31, 2018 and 2017 were as follows:



	December 31, 2018		December 31, 2017
Discount rate	1.30	%	1.31	%
Rate of compensation increase	2.83	%	3.38	%

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Income (loss) before income taxes:
United States	$	(52,569	)	$	(55,932	)	$	(11,973	)
Foreign	(20,841		)	2,240			557
	$	(73,410	)	$	(53,692	)	$	(11,416	)

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,
	2018			2017			2016
Provision (benefit) for income taxes:
Current:
Federal	$	561		$	(200	)	$	12
State	128			115			24
Foreign	2,198			1,960			1,378
Total current	2,887			1,875			1,414
Deferred:
Federal	(8,481		)	49,570			(301		)
State	(1,414		)	(4,833		)	(1,007		)
Foreign	(1,477		)	(816		)	338
Change in valuation allowance	11,885			(64,236		)	2,072
Total deferred	513			(20,315		)	1,102
Total	$	3,400		$	(18,440	)	$	2,516

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,
	2018		2017		2016
U.S. statutory income tax rate	21.0	%	35.0	%	35.0	%
State income taxes, net of federal benefit	(0.1	)	1.2		—
Foreign income taxes	(5.7	)	(0.5	)	(7.9	)
Foreign tax audit	—		—		(5.2	)
Acquisition costs	(0.3	)	(6.0	)	—
Foreign deemed dividends	(3.4	)	(3.8	)	(5.0	)
Stock-based compensation	(0.3	)	26.8		(38.9	)
Tax credits	0.6		(33.3	)	11.6
Uncertain tax positions	1.3		(1.2	)	—
NOL and credit limitations	—		(18.9	)	—
Valuation allowance	(16.1	)	29.0		(1.9	)
Goodwill amortization	0.3		(1.7	)	(6.7	)
Meals and entertainment	(0.4	)	(0.5	)	(1.4	)
Tax reform	—		8.8		—
Other, net	(1.5	)	(0.6	)	(1.6	)
Effective income tax rate	(4.6	)%	34.3	%	(22.0	)%

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,
	2018			2017
Assets:
Net operating loss carryforwards	$	76,278		$	58,624
Research and development tax credits	28,664			24,499
Deferred revenue	5,755			5,886
Accrued expenses	9,601			10,786
Inventory	4,906			5,980
Stock-based compensation	1,536			727
Fixed assets	7,716			7,452
Other temporary differences	1,943			1,556
	136,399			115,510
Valuation allowance	(101,484		)	(89,600		)
Total deferred tax assets	34,915			25,910
Liabilities:
Purchased intangible assets	(26,014		)	(17,092		)
Unremitted foreign income	(4,487		)	(3,171		)
Total deferred tax liabilities	(30,501		)	(20,263		)
Total net deferred tax assets	$	4,414		$	5,647

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	$	9,152		$	8,434
Deferred income taxes - noncurrent liabilities	(4,738		)	(2,787		)
	$	4,414		$	5,647

Schedule of Unrecognized Tax Benefits

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



	2018			2017			2016
Unrecognized tax benefits at January 1	$	4,528		$	8,969		$	8,888
Increases related to current year tax positions	74			139			36
Increases related to prior period tax positions	122			430			723
Increases related to business acquisitions	—			2,012			—
Decreases related to prior period tax positions	(1,263		)	(7,022		)	(81		)
Settlements	—			—			(597		)
Unrecognized tax benefits at December 31	$	3,461		$	4,528		$	8,969

QUARTERLY RESULTS (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2018

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			Second Quarter			Third Quarter (1)			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



	First Quarter			Second Quarter			Third Quarter		Fourth Quarter (2)
	(In thousands, except per share data)
Fiscal 2017
Revenue	$	53,368		$	55,733		$	74,629	$	146,212
Cost of revenue	19,620			19,331			20,082		69,413
Gross profit	$	33,748		$	36,402		$	54,547	$	76,799
Loss (income) from operations	$	(10,782	)	$	(12,703	)	$	3,919	$	(35,663	)
Net (loss) income	$	(10,646	)	$	(12,345	)	$	3,453	$	(15,714	)
(Loss) earnings per share (3):
Basic	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Diluted	$	(0.22	)	$	(0.25	)	$	0.07	$	(0.18	)
Shares used in computing (loss) diluted earnings per share:
Basic	49,114			49,543			49,753		86,567
Diluted	49,114			49,543			50,131		86,567

__________________________________


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


(2)	Includes the results of GENBAND for the period subsequent to October 27, 2017.


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

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 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	$ 0	$ 0	$ 0
Advertising expense	$ 500,000	$ 300,000	$ 100,000
Number of reportable operating segments \| segment	1
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

BUSINESS ACQUISITONS - EDGEWATER NETWORKS, INC (Details) - USD ($) $ in Thousands, shares in Millions		3 Months Ended	12 Months Ended
	Jun. 24, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 15, 2019
Business Acquisition [Line Items]
Cash, net of cash acquired			$ 46,389	$ 42,951	$ 20,669
Deferred purchase consideration		$ 30,000	30,000	$ 0
Decrease of current assets		700
Increase to current liabilities		500
Net increase to goodwill		$ 1,300
Edgewater
Business Acquisition [Line Items]
Cash, net of cash acquired	$ 46,400		$ 46,389
Shares issued to selling shareholders	4.2
Deferred purchase consideration	$ 30,000
Deferred cash payments, within 6 months from closing date	15,000
Deferred cash payments, within 9 to 18 months from closing date	$ 15,000
Weighted average life of identifiable intangible assets (in years)			8 years 5 months
Revenue attributable since acquisition			$ 21,500
Net loss attributable to acquisition			$ 4,300
Subsequent Event \| Scenario, Adjustment \| Edgewater
Business Acquisition [Line Items]
Deferred purchase consideration						$ 21,900

BUSINESS ACQUISITONS - SCHEDULE OF EDGEWATER PURCHASE PRICE ALLOCATION (Details) - USD ($) $ in Thousands			12 Months Ended
	Jun. 24, 2018	Oct. 27, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Net cash consideration			$ 46,389	$ 42,951	$ 20,669
Fair value of Ribbon stock issued		$ 413,982
Goodwill			383,655	$ 335,716	$ 49,393
Edgewater
Business Acquisition [Line Items]
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
Current assets			16,098
Property and equipment			245
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
Business Acquisition [Line Items]
Finite-lived intangibles			29,500
Customer relationships \| Edgewater
Business Acquisition [Line Items]
Finite-lived intangibles			26,100
Trade names \| Edgewater
Business Acquisition [Line Items]
Finite-lived intangibles			$ 1,100

BUSINESS ACQUISITONS - GENBAND MERGER (Details) - USD ($)		12 Months Ended
	Oct. 27, 2017	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Debt average interest rate		5.96%	4.51%
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash consideration:
Net cash consideration		$ 46,389,000	$ 42,951,000	$ 20,669,000
Fair value of Sonus stock issued	$ 413,982,000
Intangible assets:
Goodwill		$ 383,655,000	$ 335,716,000	$ 49,393,000
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) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Revenue	$ 615,286	$ 631,914
Net loss	$ (69,741)	$ (147,394)
Loss per share	$ (0.69)	$ (1.46)

BUSINESS ACQUISITONS - TAQUA, LLC ACQUISITION (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended			12 Months Ended
	Sep. 26, 2016	Apr. 30, 2018	Dec. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Cash, net of cash acquired						$ 46,389	$ 42,951	$ 20,669
Net reduction in goodwill			$ (1,300)
Goodwill			$ 383,655		$ 49,393	$ 383,655	$ 335,716	49,393
Taqua, LLC
Business Acquisition [Line Items]
Cash, net of cash acquired	$ 19,919
Current assets	10,000
Primary adjustments, increase to current liabilities				$ 400
Primary adjustments, increase to noncurrent liabilities				$ 100
Reversal of contingent liability					10,000
Net reduction in goodwill					2,200
Goodwill	9,581
Business combination, pro forma revenue								1,900
Earnings (loss) attributable to acquisition								$ 4,700
Taqua, LLC \| Developed technology
Business Acquisition [Line Items]
Increase (decrease) in intangible assets					(12,100)
Taqua, LLC \| Customer relationships
Business Acquisition [Line Items]
Increase (decrease) in intangible assets					$ 5,500
Taqua, LLC \| Maximum
Business Acquisition [Line Items]
Current assets	$ 65,000
Taqua Settlement Agreement
Business Acquisition [Line Items]
Release of funds held in escrow		$ 600
Payment in lieu of future payments under agreement		1,400
Total amount paid under settlement		$ 2,000

BUSINESS ACQUISITONS - SCHEDULE OF TAQUA, LLC PURCHASE PRICE ALLOCATION (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 26, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value of consideration transferred:
Cash, net of cash acquired		$ 46,389	$ 42,951	$ 20,669
Fair value of assets acquired and liabilities assumed:
Goodwill		$ 383,655	$ 335,716	$ 49,393
Taqua, LLC
Fair value of consideration transferred:
Cash, net of cash acquired	$ 19,919
Fair value of assets acquired and liabilities assumed:
Current assets	3,347
Property and equipment	1,478
Goodwill	9,581
Other noncurrent assets	23
Current liabilities	(5,435)
Long-term liabilities	(685)
Assets acquired and liabilities assumed	19,919
Taqua, LLC \| Developed technology
Fair value of assets acquired and liabilities assumed:
Intangible assets	2,100
Taqua, LLC \| Customer relationships
Fair value of assets acquired and liabilities assumed:
Intangible assets	$ 9,510

BUSINESS ACQUISITONS - SCHEDULE OF ACQUISITION- AND INTEGRATION-RELATED COSTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Professional and services fees (acquisition-related)	$ 7,627	$ 11,916	$ 1,152
Management bonuses (acquisition-related)	1,972	931	0
Integration-related expenses	7,352	1,916	0
Acquisition- and integration-related	$ 16,951	$ 14,763	$ 1,152

EARNINGS (LOSS) PER SHARE - (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of weighted average shares outstanding from basic to diluted
Weighted average shares outstanding - basic	106,607	104,918	102,160	101,917	86,567	49,753	49,543	49,114	103,916	58,822	49,385
Potential dilutive common shares									0	0	0
Weighted average shares outstanding - diluted	106,607	104,918	102,160	101,917	86,567	50,131	49,543	49,114	103,916	58,822	49,385
Common stock and unvested shares of restricted stock not included because their effect would have been antidilutive (in shares)									3,100	2,500	8,000

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Activity for Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Cash equivalents, amortized cost	$ 310	$ 1,254
Cash equivalents, fair value	310	1,254
Short-term investments, Amortized cost	7,299	17,274
Short-term investments, Unrealized gains	0	0
Short-term investments, Unrealized losses	(15)	(50)
Short-term investments, Fair value	7,284	17,224
Investments, amortized cost		9,083
Investments, unrealized gains		0
Investments, unrealized losses		(52)
Investments, fair value	$ 0	9,031
Minimum
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Period considered to classify available-for-sale securities as investments (in years)	1 year
Maximum
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Period considered to classify available-for-sale securities as investments (in years)	2 years
U.S. government agency notes
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Short-term investments, Amortized cost	$ 3,998	4,091
Short-term investments, Unrealized gains	0	0
Short-term investments, Unrealized losses	(9)	(19)
Short-term investments, Fair value	3,989	4,072
Investments, amortized cost		3,992
Investments, unrealized gains		0
Investments, unrealized losses		(28)
Investments, fair value		3,964
Corporate debt securities
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Short-term investments, Amortized cost	3,301	8,048
Short-term investments, Unrealized gains	0	0
Short-term investments, Unrealized losses	(6)	(31)
Short-term investments, Fair value	$ 3,295	8,017
Investments, amortized cost		3,908
Investments, unrealized gains		0
Investments, unrealized losses		(24)
Investments, fair value		3,884
Certificates of deposit
Amortized cost, gross unrealized gains and losses and fair value of marketable debt and equity securities and investments
Short-term investments, Amortized cost		5,135
Short-term investments, Unrealized gains		0
Short-term investments, Unrealized losses		0
Short-term investments, Fair value		5,135
Investments, amortized cost		1,183
Investments, unrealized gains		0
Investments, unrealized losses		0
Investments, fair value		$ 1,183

CASH EQUIVALENTS AND INVESTMENTS - Schedule of Cash Equivalents and Investments by Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	$ 310	$ 1,254
Marketable securities, fair value	7,284	17,224
Investments, fair value	0	9,031
Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	310	1,254
Marketable securities, fair value	0	0
Investments, fair value		0
Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	0	0
Marketable securities, fair value	7,284	17,224
Investments, fair value		9,031
Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Cash equivalents, fair value	0	0
Marketable securities, fair value	0	0
Investments, fair value		0
U.S. government agency notes
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	3,989	4,072
Investments, fair value		3,964
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	0
Investments, 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	4,072
Investments, fair value		3,964
U.S. government agency notes \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, fair value		0
Corporate debt securities
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	3,295	8,017
Investments, fair value		3,884
Corporate debt securities \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	0	0
Investments, 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	8,017
Investments, fair value		3,884
Corporate debt securities \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value	$ 0	0
Investments, fair value		0
Certificates of deposit
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		5,135
Investments, fair value		1,183
Certificates of deposit \| Quoted prices in active markets (Level 1)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
Investments, fair value		0
Certificates of deposit \| Significant other observable inputs (Level 2)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		5,135
Investments, fair value		1,183
Certificates of deposit \| Significant unobservable inputs (Level 3)
Cash and cash equivalents, Marketable securities and Investments
Marketable securities, fair value		0
Investments, fair value		$ 0

ACCOUNTS RECEIVABLE, NET - Schedule of Accounts Receivabale, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Accounts receivable	$ 188,522	$ 165,229
Allowance for doubtful accounts	(669)	(73)	$ (10)	$ (10)
Accounts receivable, net	$ 187,853	$ 165,156

ACCOUNTS RECEIVABLE, NET - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 73	$ 10	$ 10
Charges to expense	351	154	10
Charges (credits) to other accounts (deferred revenue)	620	(56)	0
Write-offs	(375)	(35)	(10)
Balance at end of year	$ 669	$ 73	$ 10

INVENTORY - (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
On-hand final assemblies and finished goods inventories	$ 19,879	$ 18,374
Deferred cost of goods sold	3,798	4,569
Gross inventory	23,677	22,943
Less noncurrent portion (included in Other assets)	(1,075)	(1,640)
Current portion	$ 22,602	$ 21,303

PROPERTY AND EQUIPMENT - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 123,840	$ 109,712
Less accumulated depreciation and amortization	(96,798)	(84,932)
Property and equipment, net	$ 27,042	24,780
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	$ 76,423	67,415
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	$ 24,707	21,977
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	$ 1,490	1,892
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	$ 18,428
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 - Property and Equipment Under Capital Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Cost	$ 2,979	$ 664
Less accumulated depreciation	(875)	(180)
Property and equipment under capital leases, net	$ 2,104	$ 484

PROPERTY AND EQUIPMENT - Property and Equipment by Geographic Area (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 27,042	$ 24,780
United States
Property, Plant and Equipment [Line Items]
Property and equipment, net	17,862	17,576
Canada
Property, Plant and Equipment [Line Items]
Property and equipment, net	4,076	1,740
Asia/Pacific
Property, Plant and Equipment [Line Items]
Property and equipment, net	3,841	3,853
Europe
Property, Plant and Equipment [Line Items]
Property and equipment, net	1,100	1,400
Other
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 163	$ 211

INTANGIBLE ASSETS AND GOODWILL (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Oct. 27, 2017	Sep. 26, 2016
Intangible Assets And Goodwill
Weighted average amortization period (years)		7 years 10 months 18 days	7 years 9 months 7 days
Cost	$ 338,150				$ 281,450
Accumulated amortization	86,759				37,036
Net carrying value	251,391				244,414
Amortization expense		$ 49,723	$ 17,112	$ 7,500
Estimated future amortization expense for intangible assets
2019	47,411
2020	46,552
2021	40,571
2022	34,156
2023	26,756
Thereafter	55,945
Total	251,391
Impairment of intangible assets		0	5,471	0
Goodwill [Roll Forward]
Goodwill				52,499	338,822
Accumulated impairment losses				(3,106)	(3,106)
Goodwill at the beginning of the period		335,716	49,393
Goodwill at the end of the period	383,655	335,716	49,393	49,393	335,716
Amortization and Impairment of Intangible Assets		$ 49,723	$ 22,583	7,500
In-process research and development
Intangible Assets And Goodwill
Cost	5,600				5,600
Accumulated amortization	0				0
Net carrying value	5,600				5,600
Developed technology
Intangible Assets And Goodwill
Weighted average amortization period (years)		6 years 10 months 27 days	6 years 10 months 25 days
Cost	182,880				153,380
Accumulated amortization	63,187				24,211
Net carrying value	119,693				129,169
Estimated future amortization expense for intangible assets
Impairment of intangible assets	5,500
Goodwill [Roll Forward]
Amortization and Impairment of Intangible Assets		$ 38,976	$ 18,358	6,038
Customer relationships
Intangible Assets And Goodwill
Weighted average amortization period (years)		9 years 5 months 10 days	9 years 3 months 26 days
Cost	146,940				120,840
Accumulated amortization	22,218				12,015
Net carrying value	124,722				108,825
Goodwill [Roll Forward]
Amortization and Impairment of Intangible Assets		$ 10,203	$ 4,145	1,462
Trade names
Intangible Assets And Goodwill
Weighted average amortization period (years)		5 years 2 months 12 days	3 years
Cost	2,000				900
Accumulated amortization	624				80
Net carrying value	1,376				820
Goodwill [Roll Forward]
Amortization and Impairment of Intangible Assets		$ 544	$ 80	$ 0
Internal use software
Intangible Assets And Goodwill
Weighted average amortization period (years)		3 years	3 years
Cost	730				730
Accumulated amortization	730				730
Net carrying value	0				$ 0
Edgewater
Goodwill [Roll Forward]
Acquisition		$ 48,053	$ 0
Goodwill at the end of the period	$ 48,053
GENBAND
Goodwill [Roll Forward]
Acquisition		0	285,825
Goodwill at the end of the period						$ 285,825
Taqua, LLC
Goodwill [Roll Forward]
Acquisition		0	498
Write-off of goodwill attributable to dissolved subsidiary		$ (114)	$ 0
Goodwill at the end of the period							$ 9,581

ACCRUED EXPENSES - (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Employee compensation and related costs	$ 42,852	$ 37,782
Professional fees	7,994	13,743
Deferred purchase consideration - Edgewater	15,000	0
Other	18,417	24,855
Total	$ 84,263	$ 76,380

RESTRUCTURING ACCRUALS - Narrative (Details) $ in Thousands	1 Months Ended	8 Months Ended	12 Months Ended
RESTRUCTURING ACCRUALS - Narrative (Details) $ in Thousands	Jul. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Oct. 27, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 17,015	$ 9,436	$ 2,740
Long-term portions of accrued restructuring			500	$ 200
Merger Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 16,134
Number of positions eliminated \| employee			275	120
Expected future cost			$ 5,000
Merger Restructuring \| Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense			14,735	$ 8,508
Merger Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense			1,399
GENBAND Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense			0	0
Restructuring liability assumed						$ 4,100
GENBAND Restructuring \| Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense			0	0
Restructuring liability assumed						3,700
GENBAND Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense			0	0
Restructuring liability assumed						$ 400
2016 Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense	$ 2,000			531	1,500
2016 Restructuring \| Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense	1,900			405
2016 Restructuring \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense	$ 100		0	126
Taqua Restructuring Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring expense		$ 1,800		753	1,200
Taqua Restructuring Initiative \| Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring expense		1,200		245	1,000
Taqua Restructuring Initiative \| Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring expense		$ 600	0	508	$ 200
Restructuring estimate \| GENBAND Restructuring
Restructuring Cost and Reserve [Line Items]
Restructuring expense			$ 900	$ 300

RESTRUCTURING ACCRUALS - Restructuring Initiatives (Details) - USD ($) $ in Thousands	1 Months Ended	8 Months Ended	12 Months Ended
	Jul. 31, 2016	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			$ 10,176
Initiatives charged to expense			17,015	$ 9,436	$ 2,740
Balance at the end of the period			979	10,176
Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			365	602
Initiatives charged to expense		$ 1,800		753	1,200
Adjustments for changes in estimate				(49)
Cash payments				(941)
Balance at the end of the period				365	602
2016 Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			95	497
Initiatives charged to expense	$ 2,000			531	1,500
Adjustments for changes in estimate				(26)
Cash payments				(907)
Balance at the end of the period				95	497
Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			7,595
Initiatives charged to expense			16,134
Adjustments for changes in estimate			(5)
Cash payments			(21,043)
Balance at the end of the period			2,681	7,595
GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			2,121	0
Liability assumed in connection with Merger			0	4,094
Initiatives charged to expense			0	0
Adjustments for changes in estimate			886	(281)
Cash payments			(2,890)	(1,692)
Balance at the end of the period			117	2,121	0
Employee severance \| Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0	384
Initiatives charged to expense		1,200		245	1,000
Adjustments for changes in estimate				(49)
Cash payments				(580)
Balance at the end of the period				0	384
Employee severance \| 2016 Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0	497
Initiatives charged to expense	1,900			405
Adjustments for changes in estimate				(26)
Cash payments				(876)
Balance at the end of the period				0	497
Employee severance \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			7,595	0
Initiatives charged to expense			14,735	8,508
Adjustments for changes in estimate			(5)	0
Cash payments			(20,415)	(913)
Balance at the end of the period			1,910	7,595	0
Employee severance \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			1,916	0
Liability assumed in connection with Merger			0	3,663
Initiatives charged to expense			0	0
Adjustments for changes in estimate			487	(158)
Cash payments			(2,403)	(1,589)
Balance at the end of the period			0	1,916	0
Facilities \| Taqua Restructuring Initiative
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			365	218
Initiatives charged to expense		$ 600	0	508	200
Adjustments for changes in estimate			0	0
Cash payments			(332)	(361)
Balance at the end of the period			33	365	218
Facilities \| 2016 Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			95	0
Initiatives charged to expense	$ 100		0	126
Adjustments for changes in estimate			0	0
Cash payments			(37)	(31)
Balance at the end of the period			58	95	0
Facilities \| Merger Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			0
Initiatives charged to expense			1,399
Adjustments for changes in estimate			0
Cash payments			(628)
Balance at the end of the period			771	0
Facilities \| GENBAND Restructuring
Restructuring Reserve [Roll Forward]
Balance at the beginning of the period			205	0
Liability assumed in connection with Merger			0	431
Initiatives charged to expense			0	0
Adjustments for changes in estimate			399	(123)
Cash payments			(487)	(103)
Balance at the end of the period			$ 117	$ 205	$ 0

DEBT - Proposed Offering of Convertible Senior Note (Details) - Convertible Senior Notes due 2023 - USD ($)	Nov. 13, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Incurred professional fees		$ 1,000,000
Convertible debt
Debt Instrument [Line Items]
Aggregate principal amount	$ 150,000,000.0
Number of days for option	30 days
Additional aggregate principal amount	$ 25,000,000.0

DEBT - Assumed Senior Secured Credit Agreement (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Oct. 27, 2017
Debt Instrument [Line Items]
Debt average interest rate	5.96%	4.51%
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Line of credit potential increases			$ 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, 2018	Dec. 31, 2017	Dec. 21, 2017
Debt Instrument [Line Items]
Outstanding debt	$ 55,000,000	$ 20,000,000
Debt interest rate	5.96%	4.51%
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
Fronting fee percentage	0.125%
Default rate percentage	2.00%
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 \| Additional Applicable Margin \| 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	$ 2,900,000
Debt interest rate	1.75%	2.00%
Swingline loan \| Line of Credit
Debt Instrument [Line Items]
Commitments from lender			$ 15,000,000

DEBT - Promissory Note (Details) - USD ($)	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

LONG-TERM LIABILITIES - Schedule of Long-Term Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Liabilities [Abstract]
Capital lease obligations	$ 2,363	$ 837
Deferred rent	3,039	1,359
Restructuring	979	10,176
Pension obligations	7,006	7,524
Taxes payable	1,818	2,079
Deferred purchase consideration	30,000	0
Other	2,425	2,544
Long-term liabilities	47,630	24,519
Current portion	(16,833)	(11,330)
Long-term liabilities, net of current portion	$ 30,797	$ 13,189

REVENUE RECOGNITION (Details) - USD ($)	3 Months Ended								12 Months Ended
REVENUE RECOGNITION (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue recognized that was previously recorded as deferred revenue									$ 84,000,000
Revenue recognized in 2020									12,000,000
Revenue recognized in 2021									5,000,000
Revenue recognized in 2022 and beyond									1,000,000
Deferred sales commissions accrued	$ 2,700,000								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	$ (166,896,000)	$ (152,468,000)	$ (137,361,000)	$ (121,180,000)	$ (146,212,000)	$ (74,629,000)	$ (55,733,000)	$ (53,368,000)
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenue	$ 577,905	$ 329,942	$ 252,591
United States
Disaggregation of Revenue [Line Items]
Total revenue	337,624	219,057	173,979
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	96,483	44,565	33,863
Japan
Disaggregation of Revenue [Line Items]
Total revenue	39,411	24,389	25,613
Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	47,254	22,546	12,531
Other
Disaggregation of Revenue [Line Items]
Total revenue	57,133	19,385	6,605
Product
Disaggregation of Revenue [Line Items]
Total revenue	279,014	181,119	146,381
Product revenue
Disaggregation of Revenue [Line Items]
Total revenue	279,014	181,119	146,381
Product revenue \| United States
Disaggregation of Revenue [Line Items]
Total revenue	169,510	121,121	100,589
Product revenue \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	37,833	23,352	21,073
Product revenue \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	23,108	10,252	12,258
Product revenue \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	30,575	14,693	8,226
Product revenue \| Other
Disaggregation of Revenue [Line Items]
Total revenue	17,988	11,701	4,235
Service revenue (maintenance)
Disaggregation of Revenue [Line Items]
Total revenue	233,966	114,735	86,995
Service revenue (maintenance) \| United States
Disaggregation of Revenue [Line Items]
Total revenue	132,282	75,040	60,760
Service revenue (maintenance) \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	46,856	17,471	10,769
Service revenue (maintenance) \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	11,234	10,282	9,831
Service revenue (maintenance) \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	12,321	5,901	3,478
Service revenue (maintenance) \| Other
Disaggregation of Revenue [Line Items]
Total revenue	31,273	6,041	2,157
Service revenue (professional services)
Disaggregation of Revenue [Line Items]
Total revenue	64,925	34,088	19,215
Service revenue (professional services) \| United States
Disaggregation of Revenue [Line Items]
Total revenue	35,832	22,896	12,630
Service revenue (professional services) \| Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	11,794	3,742	2,021
Service revenue (professional services) \| Japan
Disaggregation of Revenue [Line Items]
Total revenue	5,069	3,855	3,524
Service revenue (professional services) \| Asia/Pacific
Disaggregation of Revenue [Line Items]
Total revenue	4,358	1,952	827
Service revenue (professional services) \| Other
Disaggregation of Revenue [Line Items]
Total revenue	7,872	1,643	213
Indirect sales through channel program \| Product
Disaggregation of Revenue [Line Items]
Total revenue	69,232	43,138	38,204
Direct sales \| Product
Disaggregation of Revenue [Line Items]
Total revenue	209,782	137,981	108,177
Sales to enterprise customers \| Product
Disaggregation of Revenue [Line Items]
Total revenue	57,534	35,592	28,508
Sales to service provider customers \| Product
Disaggregation of Revenue [Line Items]
Total revenue	$ 221,480	$ 145,527	$ 117,873

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

COMMON STOCK REPURCHASES (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 29, 2013
Class of Stock [Line Items]
Payments for repurchase of common stock	$ 0	$ 0	$ 9,530,000
Common stock
Class of Stock [Line Items]
Authorized amount				$ 100,000,000
Payments for repurchase of common stock			$ 9,500,000
Shares repurchased and retired (in shares)			1.3

STOCK-BASED COMPENSATION PLANS - Amended and Restate Stock Incentive Plan (Details)	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	4,932,727
Amended and Restated Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	3,501,214

STOCK-BASED COMPENSATION PLANS - Stock Options Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 24, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from the exercise of stock options		$ 73	$ 617	$ 153
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
2008 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance under plan		0
2012 plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period (in years)		10 years
Shares available for future issuance under plan		0
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	$ 4.39
Intrinsic value of options exercised		$ 39	$ 200	$ 42
Proceeds from the exercise of stock options		100	$ 600	$ 200
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 - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted average exercise price
Granted (in dollars per share)	$ 0.00
Range of assumptions used in estimating fair value of options
Expected dividends	0.00%	0.00%	0.00%
Expected life (in years)		5 years
Stock options
Number of shares
Outstanding at the beginning of the period (in shares)	435,187
Granted (in shares)	0
Edgewater outstanding options converted to Ribbon options (in shares)	312,452
Exercised (in shares)	(15,935)
Forfeited (in shares)	(35,431)
Expired (in shares)	(114,212)
Outstanding at the end of the period (in shares)	582,061	435,187
Vested or expected to vest at the end of the period (in shares)	560,325
Exercisable at the end of the period (in shares)	428,097
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 14.71
Edgewater outstanding options converted to Ribbon options (in dollars per share)	1.97
Exercised (in dollars per share)	4.58
Forfeited (in dollars per share)	1.81
Expired (in dollars per share)	14.33
Outstanding at the end of the period (in dollars per share)	9.01	$ 14.71
Vested or expected to vest at the end of the period (in dollars per share)	9.27
Exercisable at the end of the period (in dollars per share)	$ 11.47
Weighted average remaining contractual life (in years)
Outstanding at the end of the period	4 years 9 months 27 days
Vested or expected to vest at the end of the period	4 years 8 months 2 days
Exercisable at the end of the period	3 years 7 months 5 days
Aggregate intrinsic value (in dollars)
Outstanding at the end of the period (in dollars)	$ 774
Vested or expected to vest at the end of the period (in dollars)	720
Exercisable at the end of the period (in dollars)	$ 366
Range of assumptions used in estimating fair value of options
Risk-free interest rates, minimum		1.22%	1.00%
Risk-free interest rates, maximum		1.95%	1.61%
Weighted average volatility		51.10%	54.80%

STOCK-BASED COMPENSATION PLANS - Stock Option Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividends	0.00%	0.00%	0.00%
Expected life (in years)		5 years
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (in years)			5 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life (in years)			10 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rates, minimum		1.22%	1.00%
Weighted average volatility		51.10%	54.80%
Risk-free interest rates, maximum		1.95%	1.61%

STOCK-BASED COMPENSATION PLANS - RSA and RSU Activity (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
RSUs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares) \| shares	17,932
Granted (in shares) \| shares	908,300
Vested (in shares) \| shares	(242,110)
Forfeited (in shares) \| shares	(47,822)
Unvested balance at the end of the period (in shares) \| shares	636,300
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share) \| $ / shares	$ 6.90
Granted (in dollars per share) \| $ / shares	6.27
Vested (in dollars per share) \| $ / shares	5.57
Forfeited (in dollars per share) \| $ / shares	6.76
Unvested balance at end of the period (in dollars per share) \| $ / shares	$ 6.52
RSAs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares) \| shares	1,696,582
Granted (in shares) \| shares	1,123,956
Vested (in shares) \| shares	(1,035,952)
Forfeited (in shares) \| shares	(276,575)
Unvested balance at the end of the period (in shares) \| shares	1,508,011
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share) \| $ / shares	$ 7.68
Granted (in dollars per share) \| $ / shares	6.90
Vested (in dollars per share) \| $ / shares	8.05
Forfeited (in dollars per share) \| $ / shares	7.36
Unvested balance at end of the period (in dollars per share) \| $ / shares	$ 6.90

STOCK-BASED COMPENSATION PLANS - RSA and RSU Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 8,312	$ 20,515	$ 10,376
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	$ 9,700	$ 19,100	$ 10,100

STOCK-BASED COMPENSATION PLANS - Treatment of Equity Awards in Connection with the Merger (Details) - shares shares in Thousands				12 Months Ended
	Oct. 27, 2017	Oct. 24, 2017	Oct. 20, 2017	Dec. 31, 2018
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) $ in Thousands								1 Months Ended				12 Months Ended
	Apr. 01, 2018 shares	Mar. 16, 2018 shares	Apr. 01, 2017 shares	Mar. 31, 2017 executive shares	Mar. 16, 2017 shares	Apr. 01, 2016 executive shares	Mar. 16, 2015 executive shares	Feb. 28, 2019 executive shares	May 31, 2018 shares	Mar. 31, 2018 executive shares	Feb. 28, 2017 shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $												$ 11,072	$ 25,657	$ 19,768
PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $												$ 300
Granted (in shares)						131,250	131,250		195,000			207,350
Percent of performance metrics achieved									150.00%
Vested (in shares)												57,768
Executive granted shares \| executive						6	8
Forfeited (in shares)												0
Unvested balance outstanding												210,416	60,834
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 \| Uplift Shares and 2018 Bonus Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ratio multiplier												120.00%
Stock-based compensation \| $												$ 1,100
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										130.00%
Vested (in shares)										33,584
Executive granted shares \| executive				5
Number of remaining executives holding grants \| executive										3
2016 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved										130.00%	90.40%
Vested (in shares)										16,250	24,106
Number of remaining executives holding grants \| executive										2
Forfeited (in shares)					2,560
Unvested balance outstanding												0
2015 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved										112.00%	76.00%
Vested (in shares)										7,934	23,750
Number of remaining executives holding grants \| executive										2
Forfeited (in shares)			7,500
Unvested balance outstanding												0
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%
April 1, 2016 \| 2016 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)										12,500
April 1, 2018 \| 2016 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)	3,750
Percent achievement over target	30.00%
March 16, 2015 \| 2015 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)										7,084
March 16, 2018 \| 2015 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)		850
Percent achievement over target		12.00%
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%
Subsequent Event \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved, level one								106.49%
Percent of performance metrics achieved, level two								150.00%
Vested (in shares)								250,075
Subsequent Event \| 2017 Performance Share Units \| PSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of performance metrics achieved								61.40%
Vested (in shares)								9,464
Number of remaining executives holding grants \| executive								3

STOCK-BASED COMPENSATION PLANS - Performance-Based Stock Units Activity (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Apr. 01, 2016	Mar. 16, 2015	May 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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				$ 8,312	$ 20,515	$ 10,376
PSUs
Change in unvested restricted stock awards
Unvested balance at the beginning of the period (in shares)				60,834
Granted (in shares)	131,250	131,250	195,000	207,350
Vested (in shares)				(57,768)
Forfeited (in shares)				0
Unvested balance at the end of the period (in shares)				210,416	60,834
Weighted average grant-date fair value
Unvested balance at the beginning of the period (in dollars per share)				$ 9.65
Granted (in dollars per share)				5.78
Vested (in dollars per share)				9.90
Forfeited (in dollars per share)				0.00
Unvested balance at end of the period (in dollars per share)				$ 5.77	$ 9.65
Total fair value of restricted stock awards, restricted stock units, performance-based stock awards and performance-based stock units on date vested				$ 600	$ 1,400	$ 200

STOCK-BASED COMPENSATION PLANS - ESPP (Details)	12 Months Ended
STOCK-BASED COMPENSATION PLANS - ESPP (Details)	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	4,932,727
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	500
Number of shares authorized	5,000,000
Shares available for future issuance (in shares)	1,431,513

STOCK-BASED COMPENSATION PLANS - Stock-Based Compensation (Details) - USD ($)		3 Months Ended	12 Months Ended
	Dec. 13, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit for employee stock-based compensation			$ 0	$ 0	$ 0
Stock-based compensation			11,072,000	25,657,000	19,768,000
Unrecognized stock-based compensation expense			$ 10,700,000
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				8,600,000
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 2,935,000	$ 4,885,000	$ 6,209,000
Restatement Adjustment \| Sales and marketing \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ (1,000,000)

STOCK-BASED COMPENSATION PLANS - Schedule of Stock-Based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 11,072	$ 25,657	$ 19,768
Product cost of revenue
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	114	514	359
Service cost of revenue
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	345	1,448	1,314
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	1,797	7,337	5,014
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	2,935	4,885	6,209
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 5,881	$ 11,473	$ 6,872

STOCK-BASED COMPENSATION PLANS - Common Stock Reserved (Details)	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	4,932,727
Amended and Restated Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	3,501,214
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	1,431,513

EMPLOYEE DEFINED CONTRIBUTION PLANS - (Details) - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Maximum amount of employer match per employee		$ 2,000
Meximum semi-annual amount of employer match per employee		1,000
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		$ 3,200,000	$ 1,400,000	$ 600,000

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Narrative (Details) - USD ($)	2 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Oct. 27, 2017
Retirement Benefits [Abstract]
Curtailment gain		$ 500,000
Benefits paid	$ 3,000	273,000
One-time lump sum payments to participants		$ 250,000
Net gains and losses amortization threshold		10.00%
Expected amortization of gain (loss) in the next fiscal year		$ 4,000
Fair value of combined plan assets	3,893,000	3,842,000	$ 3,776,000
Participant contributions	$ 5,000	5,000
Fixed contributions per employee, percent	5.00%
Employer pension plan contributions	$ 22,000	292,000
Expected future employer contributions to pension plans in 2019		$ 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, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of period	$ 10,515	$ 11,484
Service cost	68	449
Interest cost	25	150
Participant contributions	5	5
Benefits paid	(3)	(23)
Net actuarial loss on obligation	562	(414)
Curtailment	0	(553)
Settlement	0	(250)
Currency loss	312	0
Projected benefit obligation, end of year	11,484	10,848
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	3,776	3,893
Actual return on plan assets	(8)	(53)
Employer contributions	22	292
Participant contributions	5	5
Administrative expenses	(4)	(22)
Benefits paid	(3)	(273)
Currency gain	105	0
Fair value of plan assets, end of year	3,893	3,842
Funded status at end of year	(7,591)	(7,006)
Net actuarial loss	578	222
Liability, Defined Benefit Plan [Abstract]
Accrued expenses and other (current pension liability)	(67)	(75)
Other long-term liabilities (non-current pension liability)	(7,524)	(6,931)
Net amount recognized	$ (7,591)	$ (7,006)

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Oct. 27, 2017
Retirement Benefits [Abstract]
Aggregate projected benefit obligation	$ 10,848	$ 11,484
Aggregate accumulated benefit obligation	7,152	7,793
Aggregate fair value of plan assets	$ 3,842	$ 3,893	$ 3,776

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, 2018
Retirement Benefits [Abstract]
Service cost	$ 68	$ 449
Interest cost	25	150
Expected return on plan assets	(8)	(45)
Additional charges	4	22
Curtailment gain	0	(510)
Settlement loss	0	3
Net periodic benefit costs	$ 89	$ 69

NON-U.S. EMPLOYEE DEFINED BENEFIT PLANS - Expected Future Benefit Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Retirement Benefits [Abstract]
2019	$ 75
2020	45
2021	94
2022	47
2023	231
2024 to 2028	643
Expected future benefit payments	$ 1,135

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, 2018
Retirement Benefits [Abstract]
Discount rate	1.49%	1.50%
Expected long-term return on plan assets	1.23%	1.34%
Rate of compensation increase	3.38%	3.38%

INCOME TAXES - Schedule of Income (Loss) Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss before income taxes:
United States	$ (52,569)	$ (55,932)	$ (11,973)
Foreign	(20,841)	2,240	557
Loss before income taxes	$ (73,410)	$ (53,692)	$ (11,416)

INCOME TAXES - Schedule of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ 561	$ (200)	$ 12
State	128	115	24
Foreign	2,198	1,960	1,378
Total current	2,887	1,875	1,414
Deferred:
Federal	(8,481)	49,570	(301)
State	(1,414)	(4,833)	(1,007)
Foreign	(1,477)	(816)	338
Change in valuation allowance	11,885	(64,236)	2,072
Total deferred	513	(20,315)	1,102
Total	$ 3,400	$ (18,440)	$ 2,516

INCOME TAXES - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. statutory income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal benefit	(0.10%)	1.20%	0.00%
Foreign income taxes	(5.70%)	(0.50%)	(7.90%)
Foreign tax audit	0.00%	0.00%	(5.20%)
Acquisition costs	(0.30%)	(6.00%)	0.00%
Foreign deemed dividends	(3.40%)	(3.80%)	(5.00%)
Stock-based compensation	(0.30%)	26.80%	(38.90%)
Tax credits	0.60%	(33.30%)	11.60%
Uncertain tax positions	1.30%	(1.20%)	0.00%
NOL and credit limitations	0.00%	(18.90%)	0.00%
Valuation allowance	(16.10%)	29.00%	(1.90%)
Goodwill amortization	0.30%	(1.70%)	(6.70%)
Meals and entertainment	(0.40%)	(0.50%)	(1.40%)
Tax reform	0.00%	8.80%	0.00%
Other, net	(1.50%)	(0.60%)	(1.60%)
Effective income tax rate	(4.60%)	34.30%	(22.00%)

INCOME TAXES - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Net operating loss carryforwards	$ 76,278	$ 58,624
Research and development tax credits	28,664	24,499
Deferred revenue	5,755	5,886
Accrued expenses	9,601	10,786
Inventory	4,906	5,980
Stock-based compensation	1,536	727
Fixed assets	7,716	7,452
Other temporary differences	1,943	1,556
Deferred tax assets, gross	136,399	115,510
Valuation allowance	(101,484)	(89,600)
Total deferred tax assets	34,915	25,910
Liabilities:
Purchased intangible assets	(26,014)	(17,092)
Unremitted foreign income	(4,487)	(3,171)
Total deferred tax liabilities	(30,501)	(20,263)
Total net deferred tax assets	4,414	5,647
Deferred income taxes - noncurrent assets	9,152	8,434
Deferred income taxes - noncurrent liabilities	$ (4,738)	$ (2,787)

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 24, 2018	Jan. 01, 2018
Operating Loss Carryforwards [Line Items]
Deferred tax assets, stock-based compensation	$ 1,536	$ 727
Tax credit carryforward	$ 28,700
Significant change in shareholder ownership, period of change	3 years
Significant change in shareholder ownership, benchmark percentage	50.00%
Decrease in deferred tax assets	$ (9,152)	(8,434)
BEAT tax expense	400
Income tax benefit	(3,400)	18,440	$ (2,516)
Deferred tax assets, valuation allowance	101,484	89,600
Decrease in valuation allowance		200
Potential penalties and interest	100	200	100
Interest and penalties accrued in deferred tax liability	600	600	$ 200
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	289,100
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	82,400	73,100
Edgewater
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards acquired				$ 34,000
Tax credit carryforwards acquired				$ 6,000
Income tax benefit	700
GENBAND
Operating Loss Carryforwards [Line Items]
Income tax benefit		16,400
Deferred tax assets, valuation allowance	9,600
IRELAND \| GENBAND
Operating Loss Carryforwards [Line Items]
Deferred tax assets, valuation allowance	$ 9,500	$ 6,200
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at January 1	$ 4,528	$ 8,969	$ 8,888
Increases related to current year tax positions	74	139	36
Increases related to prior period tax positions	122	430	723
Increases related to business acquisitions	0	2,012	0
Decreases related to prior period tax positions	(1,263)	(7,022)	(81)
Settlements	0	0	(597)
Unrecognized tax benefits at December 31	$ 3,461	$ 4,528	$ 8,969

MAJOR CUSTOMERS - (Details) - Customer	12 Months Ended
MAJOR CUSTOMERS - (Details) - Customer	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue \| Verizon Communications Inc.
MAJOR CUSTOMERS
Concentration risk, percentage	17.00%	17.00%
Revenue \| AT&T Inc.
MAJOR CUSTOMERS
Concentration risk, percentage			12.00%
Accounts receivable
MAJOR CUSTOMERS
Concentration risk, percentage	32.00%	31.00%

RELATED PARTIES (Details) - USD ($)	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

COMMITMENTS AND CONTINGENCIES - Leases Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Area of leased facility (in sqft) \| ft²	97,500
Rent expense	$ 11,900	$ 5,900	$ 4,500
Years ending December 31,
2019	10,705
2020	8,384
2021	7,455
2022	5,691
2023	5,430
Thereafter	19,818
Operating lease, future minimum payments	$ 57,483

COMMITMENTS AND CONTINGENCIES - Litigation and Contingencies (Details) $ in Millions				12 Months Ended
	Apr. 10, 2018 USD ($)	Mar. 22, 2018 USD ($)	Jan. 15, 2016 USD ($)	Dec. 31, 2018 USD ($) lawsuit
Loss Contingencies [Line Items]
Ongoing lawsuits \| lawsuit				6
Damages awarded to other party			$ 8.2
Damages awarded from other party		$ 8.9
Awarded judgment interests and costs	$ 0.4
Unfavorable Regulatory Action
Loss Contingencies [Line Items]
Civil penalty				$ 1.9

SUBSEQUENT EVENT (Details) shares in Millions	Feb. 28, 2019 shares
Anova Data, Inc. \| Subsequent Event
Subsequent Event [Line Items]
Shares issued as consideration in connection with acquisition (in shares)	3.3

QUARTERLY RESULTS (UNAUDITED) - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 166,896	$ 152,468	$ 137,361	$ 121,180	$ 146,212	$ 74,629	$ 55,733	$ 53,368
Cost of revenue	71,182	70,234	62,250	65,907	69,413	20,082	19,331	19,620
Gross profit	95,714	82,234	75,111	55,273	76,799	54,547	36,402	33,748	$ 308,332	$ 201,496	$ 167,611
(Loss) income from operations	1,177	(7,566)	(16,636)	(42,383)	(35,663)	3,919	(12,703)	(10,782)	(65,408)	(55,229)	(13,609)
Net loss	$ (1,826)	$ (10,158)	$ (19,922)	$ (44,904)	$ (15,714)	$ 3,453	$ (12,345)	$ (10,646)	$ (76,810)	$ (35,252)	$ (13,932)
Loss per share:
Basic (in dollars per share)	$ (0.02)	$ (0.10)	$ (0.20)	$ (0.44)	$ (0.18)	$ 0.07	$ (0.25)	$ (0.22)	$ (0.74)	$ (0.60)	$ (0.28)
Diluted (in dollars per share)	$ (0.02)	$ (0.10)	$ (0.20)	$ (0.44)	$ (0.18)	$ 0.07	$ (0.25)	$ (0.22)	$ (0.74)	$ (0.60)	$ (0.28)
Shares used in computing loss per share:
Basic (in shares)	106,607	104,918	102,160	101,917	86,567	49,753	49,543	49,114	103,916	58,822	49,385
Diluted (in shares)	106,607	104,918	102,160	101,917	86,567	50,131	49,543	49,114	103,916	58,822	49,385

COMMON STOCK REPURCHASES	12 Months Ended
COMMON STOCK REPURCHASES	Dec. 31, 2018
Equity [Abstract]
COMMON STOCK REPURCHASES	COMMON STOCK REPURCHASES On July 29, 2013, Sonus announced that its Board of Directors had authorized a stock buyback program to repurchase up to $100 million of its common stock from time to time on the open market or in privately negotiated transactions. The timing and amount of any shares repurchased was determined by Sonus' management based on its evaluation of market conditions and other factors. The buyback program did not have a fixed expiration date but could be suspended or discontinued at any time. The buyback program was funded using Sonus' working capital. Ribbon did not assume the stock buyback program in connection with the Merger. During the years ended December 31, 2018 and 2017, the Company did not repurchase any shares under the stock buyback program. During the year ended December 31, 2016, Sonus spent $9.5 million, including transaction fees, to repurchase and retire 1.3 million shares of its common stock under the buyback program.

SUBSEQUENT EVENT	12 Months Ended
SUBSEQUENT EVENT	Dec. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENT	SUBSEQUENT EVENT On February 28, 2019, the Company acquired the business and technology assets of Anova Data, Inc. ("Anova"), a provider of advanced analytics solutions, for total purchase consideration of 3.3 million shares of the Company's common stock. Anova is based in the United States and its NextGen products provide a cloud-native, streaming analytics platform for network and subscriber optimization and monetization. The Company believes that the proposed acquisition reinforces and extends Ribbon's strategy to expand into network optimization, security and data monetization via big data analytics and machine learning. The Company does not expect that the acquisition will have a material effect on its consolidated financial statements in 2019.

NATURE OF THE BUSINESS (Narrative) (Details) customer in Thousands	Dec. 31, 2018 customer
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of customers	1

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Foreign Currency Translation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
General and administrative
Schedule of Foreign Currency Balance [Line Items]
Transaction gains (losses)	$ (4.6)	$ 0.7	$ (0.3)

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting for Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Tax liability related to undistributed foreign earnings	$ 4,487	$ 3,171