Kamailio

Kamailio is an open source SIP server used to build large platforms for VoIP and realtime communications, including presence, WebRTC and instant messaging.

It occupies a different position from the PBX products it is usually listed beside. Kamailio is a SIP proxy and registrar rather than a phone system, handling signalling, routing and scale, and it is what sits in front of a fleet of media servers when a platform outgrows what one PBX can carry. Operators and large VoIP providers use it as infrastructure rather than as the thing users interact with.

The feature list reflects that role. Transport covers asynchronous TCP, UDP and SCTP with TLS for secure voice, video and text, WebSocket support for WebRTC, and both IPv4 and IPv6. Telephony features include SIMPLE instant messaging and presence with an embedded XCAP server and MSRP relay, IMS extensions for VoLTE, ENUM, DID and least cost routing, load balancing, routing failover, and accounting, authentication and authorization.

Backend flexibility is unusually broad, supporting MySQL, Postgres, Oracle, Radius, LDAP, Redis, Cassandra, MongoDB and Memcached, with JSON and XMLRPC control interfaces. That range matters at scale because the subscriber database and the routing store are frequently already chosen by the wider platform, and a SIP server that cannot talk to them is not usable. Kamailio is free and open source.