The Cisco® SIP Proxy Server is a software application that provides call-routing services in an IP telephony network. Using the Cisco SIP Proxy Server, service providers and enterprises can create large-scale, highly reliable packet voice networks.

Session Initiation Protocol (SIP) is an Internet Engineering Task Force (IETF) multimedia communications standard. The Cisco SIP Proxy Server combines the standard functions of a SIP proxy server and a SIP registrar with additional features to create an IP telephony infrastructure component.

The Cisco SIP Proxy Server is part of the Cisco Voice Infrastructure and Applications (VIA) solution for service providers. Service providers can use the Cisco SIP Proxy Server to offer voice services between SIP-based application service providers (ASPs) and the public switched telephone network (PSTN). The Cisco SIP Proxy Server also provides residential voice services over broadband access as part of the Cisco Broadband Local Integrated Services Solution (BLISS). Enterprises can use the Cisco SIP Proxy Server for internal IP telephony transit among enterprise voice gateways and SIP-capable IP PBXs, as well as interconnection with the PSTN.

http://www.cisco.com/en/US/products/sw/voicesw/ps2157/index.html