update : 23/12/2020 Kamailio

Kamailio is an open source SIP server. This fork of the OpenSER project (in 2005) is one of the most complete PBXs.

It supports asynchronous TCP, UDP and SCTP transactions, communications encryptions via TLS, load balancing, a native failover mechanism, authentication on Radius, MySQL and LDAP back ends or via XML-RCP transport. It is used both by telecoms operators, as a VoIP platform, and in traditional corporate telephony solutions. It is an alternative to FreeSWITCH and Asterisk, the two other heavyweights in this domain.

Kamailio is distributed under the GPL.