<Page> 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 SEPTEMBER 23, 2003 ---------- Date of Report (Date of earliest event reported) ---------- SONUS NETWORKS, INC. ---------- (Exact Name of Registrant as Specified in Charter) DELAWARE 000-30229 04-3387074 - -------------------------------------------------------------------------------- (State or Other Jurisdiction of (Commission (IRS Employer Incorporation) File Number) Identification No.) 5 CARLISLE ROAD, WESTFORD, MA 01886 ------------------------------------- (Address of Principal Executive Offices) (Zip Code) (978) 692-8999 (Registrant's telephone number, including area code) ---------- <Page> ITEM 5. OTHER EVENTS. On September 23, 2003, Sonus Networks, Inc. (the "Company") announced the pricing of a public offering of 17,000,000 shares of its common stock at $7.75 per share less applicable underwriter discounts. The Company expects the issuance and delivery of the shares to occur on September 26, 2003. Goldman, Sachs & Co. acted as sole underwriter for the offering. The Underwriting Agreement, dated as of September 23, 2003, by and between the Company and Goldman, Sachs & Co. and the Company's press release announcing such offering are filed as exhibits to this report. ITEM 7. FINANCIAL STATEMENTS, PRO FORMA FINANCIAL INFORMATION AND EXHIBITS. (c) Exhibits. EXHIBIT NUMBER DESCRIPTION Exhibit 1.1 Underwriting Agreement, dated September 23, 2003, by and between Sonus Networks, Inc. and Goldman, Sachs & Co. Exhibit 99.1 Press Release, dated September 23, 2003. <Page> SIGNATURES ---------- 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. Date: September 23, 2003 SONUS NETWORKS, INC. By: /s/ Hassan M. Ahmed ------------------------------------ Hassan M. Ahmed President and Chief Executive Officer