Kamailio is an open source SIP server application formerly named OpenSER. Starting with v3.0.0 it has merged its ancestor project, respectively SIP Express Router (SER). The project is managed by its community, released under GPLv2, and focusing to build a flexible and rock solid SIP server.

Among features: full RFC3261 implementation, support for IPv4, IPv6, UDP, TCP, TLS, SCTP, DNS SRV and NAPTR, Websockets for WebRTC, least cost routing, load balancing, AAA and prepaid billing, presence server, RCS/RCSe, embedded XACP server and MSRP relay, RADIUS, XMLRPC control interface, IMS extensions, MySQL, Postgres and many other DB backends, embedded interpreters for Lua, Perl, Python, C# and other languages, IMS extensions.