Kamailio

Kamailio is a SIP server that routes, proxies, and registers SIP traffic for VoIP, video, instant messaging, and presence. It handles thousands of call setups per second and is built to scale up to carrier and operator platforms, while remaining usable for smaller enterprise and personal deployments.

A configuration script controls call routing, and a large set of modules adds features such as load balancing, least cost routing, NAT traversal, IMS and VoLTE support, and presence. Transports include UDP, TCP, SCTP, TLS, and WebSocket for WebRTC, with IPv4 and IPv6.

Kamailio is not a turnkey SBC on its own; it is the signaling engine you use to build SBCs, edge proxies, registrars, and SIP firewalls. It runs on Linux from source or deb and rpm packages, and is licensed mainly under GPLv2 with some parts under BSD.