Vistula Communications Leverages Sonus' IMS-based Architecture to Support Reliable, Flexible VoIP Service

CHELMSFORD, Mass., September 6, 2005 - Sonus Networks, Inc. (Nasdaq: SONS), a leading supplier of service provider voice over IP (VoIP) infrastructure solutions, announced today that Vistula Communications Services, Inc. (OTCBB: VSTL) has selected Sonus' IMS-based architecture as the foundation for its next generation voice network. Vistula, which provides hosted and managed VoIP services to carriers, service providers and transit network operators throughout the world, selected Sonus because of its established track record of supplying scalable, carrier-grade solutions that facilitate the rapid development and deployment of next generation services.

Vistula is deploying a full suite of Sonus' IMS-based solutions including the Sonus GSX9000™ Open Services Switch, PSX™ Call Routing Server, SGX™ Signaling Gateway, and the Sonus Insight™ Management System. In addition, Vistula is utilizing Sonus' Network Border Switch (NBS) to manage traffic between VoIP networks.

“Vistula has long been a proponent of IP telephony solutions based on Session Initiation Protocol (SIP). Sonus is an industry pioneer in the use of SIP for the delivery of applications, and by implementing their solution, we can continue to offer our customers top of the line new services,” said Rupert Galliers-Pratt, chairman and CEO, Vistula. “Sonus' IMS-ready network architecture enables us to rapidly deliver the types of new services that carriers can leverage to establish new revenue streams with minimal capital investment.”

“Many IP telephony offers currently rely on the legacy H.323 protocol, but it is SIP that really raises the bar and enables the rapid integration of robust services that will differentiate the communication experience from traditional telephony offerings,” said Lynda Starr, research analyst with Frost and Sullivan. “Sonus was one of the industry's earliest advocates of building networks that leverage the power and simplicity of SIP, and their solutions have been in deployment by carriers and network operators for the past four years.”

Vistula's flagship IP telephony service, V-Cube™, offers both network operators and enterprises access to services such as IP Centrex, voice, video, conferencing, call center, IVR, multi/sub-domain and messaging. Because of Sonus' open-standards and distributed design, Vistula will be able to incorporate new codecs and features, which are a key part of Vistula's value proposition.

“In today's competitive market environment, carriers and network operators need to distinguish themselves by offering services that facilitate new methods of communication,” said Hassan Ahmed, chief executive officer of Sonus Networks. “Vistula's V-Cube IP telephony service enables carriers to cost-effectively leverage everything from simple voice services to video conferencing, and by building its network on a Sonus-based IMS ready architecture, Vistula is in the unique position of having a cutting-edge network architecture today, as well as a viable roadmap for the future evolution of communications services.”

About Vistula Communications Services, Inc.

Vistula Communications Services Inc (OTCBB:VSTL) is a supplier of Internet enabled voice services for telcos and service providers around the world. The company's core product, V-Cube™, is a uniquely featured IPBX that facilitates full contact communication services including Voice, Video, Conferencing, Call Centre, IVR, Multi/Sub-Domain and Unified Messaging across enterprise and public networks. Utilizing V-Cube™, Vistula offers a tailored software set that enables carriers, service providers and transit network operators to rapidly roll out a flexible range of managed VoIP based product and service offerings to their customers. The Company is headquartered in New York with overseas offices in London and Johannesburg. For more information, see www.vistula.com.

About Sonus Networks

Sonus Networks, Inc. is a leading provider of voice over IP (VoIP) infrastructure solutions for wireline and wireless service providers. With its comprehensive IP Multimedia Subsystem (IMS) solution, Sonus addresses the full range of carrier applications, including residential and business voice services, wireless voice and multimedia, trunking and tandem switching, carrier interconnection and enhanced services. Sonus' voice infrastructure solutions are deployed in service provider networks worldwide. Founded in 1997, Sonus is headquartered in Chelmsford, Massachusetts of U.S. Additional information on Sonus is available at http://www.sonusnet.com.

This release may contain forward-looking statements regarding future events that involve risks and uncertainties. Readers are cautioned that these forward-looking statements are only predictions and may differ materially from actual future events or results. Readers are referred to the “Risk Factors” section of Sonus' Annual Report on Form 10-K, dated March 15, 2005, and the “Cautionary Statements” section of Sonus' Quarterly Report on Form 10-Q, dated August 8, 2005, both filed with the SEC, which identify important risk factors that could cause actual results to differ from those contained in the forward-looking statements. Risk factors include among others: the impact of material weaknesses in our disclosure controls and procedures and our internal control over financial reporting on our ability to report our financial results timely and accurately; the unpredictability of our quarterly financial results; risks associated with our international expansion and growth; consolidation in the telecommunications industry; and potential costs resulting from pending securities litigation against the company. Any forward-looking statements represent Sonus' views only as of today and should not be relied upon as representing Sonus' views as of any subsequent date. While Sonus may elect to update forward-looking statements at some point, Sonus specifically disclaims any obligation to do so.

The information in this press release is for informational purposes only and is subject to change at Sonus' sole discretion without notice. Sonus has no obligation or commitment to develop or deliver any future release, upgrade, feature, enhancement or function described in this release. The information is provided “AS IS,” with all faults, and without any warranties whatsoever, express or implied, including, but not limited to, warranties of merchantability, performance, or fitness for a particular purpose.

Sonus is a registered trademark of Sonus Networks, Inc. Sonus GSX9000 Open Services Switch, PSX Call Routing Server, SGX Signaling Gateway, and the Sonus Insight Management System are trademarks of Sonus Networks, Inc. All other company and product names may be trademarks of the respective companies with which they are associated.