OpenSIPS

OpenSIPS is a SIP proxy and server for voice, video, instant messaging, and presence. It is a multi-purpose signaling engine used by carriers, telecoms, and ITSPs to build platforms such as Class 4 and 5 residential systems, trunking and wholesale, virtual PBX, session border controllers, load balancers, and IMS.

Routing logic is written in a configuration script, and more than 120 modules add SIP handling, backend integration, load balancing, NAT traversal, and B2BUA behavior. It reports throughput of tens of thousands of calls per second and millions of simultaneous calls, with TLS and TCP transport.

OpenSIPS is a SIP server you use to build SBCs and other platforms rather than a turnkey SBC by itself. It started as a fork of the Fraunhofer Fokus SIP Express Router and is maintained by OpenSIPS Solutions. It runs self-hosted on Linux and is licensed under the GPL.