Kamailio is an open source SIP proxy server that is capable of
handling thousands of up calls in a second. Among the features
it provides, are support for TCP, UDP and SCTP, secure communication
via TLS for VoIP (voice, video), accounting, the most popular open
source databases and much more.

Originally starting out as the SIP Express Router (SER) project by
the Fraunhofer Society in 2001, the design team got together in
2008, merged old and new source code and rebranded SER to Kamailio.

WWW: https://www.kamailio.org/