KAMAILIO (OpenSER) is a mature and flexible open source SIP server (RFC3261).
It can be used on systems with limitted resources as well as on carrier grade
servers, scaling to up to thousands call setups per second. It is written in
pure C for Unix/Linux-like systems with architecture specific optimizations to
offer high performances. It is customizable, being able to feature as fast load
balancer; SIP server flavours: registrar, location server, proxy server,
redirect server; gateway to SMS/XMPP; or advanced VoIP application server.
