Kamailio

Complete, modular and scalable SIP server, includes a push gateway, to deliver SIP incoming calls...

Easy to use but advanced IP PBX system, VoIP gateway and conference server

Lightweight SIP proxy, location server, and registrar for a reliable and scalable SIP infrastructure

SIP proxy/server for voice, video, IM, presence and any other SIP extensions

HTTP and reverse proxy server, mail proxy server, and generic TCP/UDP proxy server

Scalable open source cross-platform telephony platform