OpenSIPS
OpenSIPS is simply an acronym of Open Session Initiation Protocol Server, which is an open source implementation of a GPLed SIP server. Open source simply means that a product is completely accessible from a design and implementation perspective, and can be distributed free of charge. OpenSIPS is an open source SIP proxy/server known for its high capacity and robustness. OpenSIPS can be used to support VoIP services that route millions of simultaneous calls.