UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): June 21, 2013

SONUS NETWORKS, INC.

(Exact Name of Registrant as Specified in its Charter)

4 TECHNOLOGY PARK DRIVE, WESTFORD, MASSACHUSETTS 01886

(Address of Principal Executive Offices) (Zip Code)

(978) 614-8100

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions ( see General Instruction A.2. below):

o  Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o  Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o  Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 1.01.                                        Entry into a Material Definitive Agreement.

On June 21, 2013, a committee of the Board of Directors of Sonus Networks, Inc. (the “Company”) approved and the Company entered into an amendment (the “Amendment”) to the Rights Agreement by and between the Company and American Stock Transfer & Trust Company, LLC, as Rights Agent, dated as of June 26, 2008, as amended by Amendment No. 1 to Rights Agreement dated June 10, 2011 (the “Rights Agreement’), to extend the expiration date of the rights contained therein to June 26, 2015.  The Rights Agreement remains otherwise unmodified.

A copy of the Rights Agreement and a summary of its material terms were filed with the Securities and Exchange Commission on a Form 8-K on June 27, 2008.  The foregoing description of the Amendment does not purport to be complete and is qualified in its entirety by reference to the Amendment, a copy of which is filed herewith as Exhibit 4.3 and is incorporated herein by reference.

On June 24, 2013, the Company issued a press release announcing the Amendment.  A copy of the press release is included as Exhibit 99.1 hereto.

Item 3.03.                                        Material Modification to Rights of Security Holders.

Please see the disclosure set forth under “Item 1.01. Entry into a Material Definitive Agreement,” which is incorporated by reference into this Item 3.03.

Item 9.01.                                        Financial Statements and Exhibits.

(d)

2

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

3

Exhibit Index

4

Exhibit 4.3

SONUS NETWORKS, INC.

and

AMERICAN STOCK TRANSFER & TRUST COMPANY, LLC,

as Rights Agent

AMENDMENT No. 2

TO

RIGHTS AGREEMENT

Effective as of June 21, 2013

This Amendment No. 2 to the Rights Agreement dated as of June 21, 2013 (the “Amendment”) between Sonus Networks, Inc., a Delaware corporation (the “Company”), and American Stock Transfer & Trust Company, LLC, a New York limited liability trust company, as Rights Agent (the “Rights Agent”).

WHEREAS, the Company and the Rights Agent are parties to a Rights Agreement, dated as of June 26, 2008, as amended on June 10, 2011 (the “Rights Agreement”); and

WHEREAS, the Rights Agreement, under its current terms, will expire on June 26, 2013; and

WHEREAS, pursuant to Section 27 of the Rights Agreement, the Company may amend any provision of the Rights Agreement in accordance with the terms of such Section 27; and

WHEREAS, the Board of Directors of the Company has delegated the power and authority to consider and approve an amendment to the Rights Agreement that would provide for an extension thereof to the Nominating and Corporate Governance Committee (the “Committee”); and

WHEREAS, the Committee has determined that it would be in the best interests of the Company and its stockholders to extend the Final Expiration Date (as such term is defined in the Rights Agreement) for a period of two (2) years to June 26, 2015; and

WHEREAS, the Committee has determined that it is in the best interests of the Company and its stockholders for the Company to enter into this Amendment; and

WHEREAS, for purposes of this Amendment, capitalized terms not otherwise defined herein shall have the respective meanings set forth in the Rights Agreement.

NOW THEREFORE, in consideration of the premises and mutual agreements herein set forth, the parties hereby agree as follows:

1.                                      Amendments.

1.1                               Section 7 of the Rights Agreement is amended to replace the reference to “June 26, 2013” with “June 26, 2015”.

1.2                               All references to the date of “June 26, 2013” in Exhibit B (the Form of Right Certificate) and Exhibit C (the Summary of Rights to Purchase Shares of Preferred Stock of Sonus Networks, Inc.) to the Rights Agreement shall hereby be changed to “June 26, 2015”.

2.                                      Benefits.  Nothing in this Amendment shall be construed to give to any Person other than the Company, the Rights Agent and the registered holders of the Right Certificates (and, prior to the Distribution Date, the Common Stock) any legal or equitable right, remedy or claim under this Amendment; but this Amendment shall be for the sole and exclusive benefit of the Company, the Rights Agent and the registered holders of the Right Certificates (and, prior to the Distribution Date, the Common Stock).

3.                                      Severability.  If any term, provision, covenant or restriction of this Amendment is held by a court of competent jurisdiction or other authority to be invalid, void or unenforceable, the remainder of the terms, provisions, covenants and restrictions of this Amendment shall remain in full force and effect and shall in no way be affected, impaired or invalidated.

4.                                      Descriptive Headings.  Descriptive headings of the several Sections of this Amendment are inserted for convenience only and shall not control or affect the meaning or construction of any of the provisions hereof.

5.                                      Governing Law.  This Amendment, the Rights Agreement and each Right Certificate issued thereunder shall be deemed to be a contract made under the laws of the State of Delaware and for all purposes shall be governed by and construed in accordance with the laws of such State applicable to contracts to be made and performed entirely within such State.

6.                                      Counterparts.  This Amendment may be executed in any number of counterparts and each of such counterparts shall for all purposes be deemed to be an original, and all such counterparts shall together constitute but one and the same instrument.

7.                                      Effect of Amendment.  Except as expressly modified by this Amendment, the Rights Agreement and its exhibits shall remain in full force and effect.

[Remainder of page intentionally left blank]

2

IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly executed, all as of the day and year first above written.

[SIGNATURE PAGE TO AMENDMENT NO.2 TO RIGHTS AGREEMENT]

Exhibit 99.1

SONUS NETWORKS Extends Rights Agreement

June 24, 2013 — Sonus Networks, Inc. (Nasdaq: SONS), a global leader in SIP-based communications, announced today that a committee of the Board of Directors has determined to extend the Company’s Rights Agreement designed to protect the Company from certain unsolicited takeover proposals.

At the meeting of a committee of the Board of Directors held on June 21, 2013, the Rights Agreement was amended to extend the expiration date from June 26, 2013 to June 26, 2015.  The amendment was not in response to any acquisition proposal and no other amendments were made to the Rights Agreement.

About Sonus Networks

Sonus Networks, Inc. helps the world’s leading communications service providers and enterprises embrace the next generation of SIP-based solutions including VoIP, video and Unified Communications through secure, reliable and scalable IP networks.  With customers around the globe and over 15 years of experience transforming networks to IP, Sonus has enabled service providers to capture and retain users and both service providers and enterprises to generate significant ROI.  Sonus products include session border controllers, policy/routing servers, subscriber feature servers and media and signaling gateways.  Sonus products are supported by a global services team with experience in design, deployment and maintenance for some of the world’s largest and most complex IP networks.  For more information, visit www.sonus.net or call 1-855-GO-SONUS.

For more information please contact:

Patti Leahy

Sonus Networks

978-614-8440

pleahy@sonusnet.com