FreeSwitch

FreeSwitch is a scalable open source cross-platform telephony platform designed to route and interconnect popular communication protocols using audio, video, text or any other form of media.  FreeSWITCH offers support to various stable telephony platform on which many telephony applications can be developed using a wide range of free tools. It was originally designed and implemented by focus on several design goals including modularity, cross-platform support, scalability and stability. FreeSWITCH offers many advanced SIP features such as presence/BLF/SLA as well as TCP TLS and sRTP. It can also be used as a transparent proxy with and without media in the path to act as a SBC (session border controller) and proxy T.38 and other end to end protocols.