Kamailio
Kamailio, formerly OpenSER (and sharing some common history with SIP Express Router (SER)), is a SIP server licensed under the GPL-2.0-or-later license. It can be configured to act as a SIP registrar, proxy or redirect server, and features presence support, RADIUS/syslog accounting and authorization, XML-RPC and JSON-RPC-based remote control, SQL and NoSQL backends, IMS/VoLTE extensions and others. Kamailio is a Hawaiian word. Kama'ilio means talk, to converse. "It was chosen for its special flavour." Features. Kamailio is written in pure C with architecture-specific optimizations; it can be configured for many scenarios  including small-office use, enterprise PBX replacements and carrier services—it is SIP signaling server—a proxy—aiming to be used for large real-time communication services. Features include: Usage. Kamailio is used by large Internet Service Providers to provide public telephony service. The largest public announced deployment with several million of users is in operation at the German ISP "1&1". Another large deployment is in operation at the provider sipgate. Forks. OpenSIPS. OpenSIPS, a fork of SER which has diverged—deciding to "go their own way" from the SER and OpenSER codebases—is a free software implementation of SIP  for voice over IP (VoIP) that can be used to handle voice, text and video communication. OpenSIPS is intended for installations serving thousands of calls and is IETF RFC 3261 compliant. The software was recognized by Google in 2017 with their Open Source Peer Bonus award. History. Kamailio's roots go back to 2001, when the first line of SIP Express Router (SER) was written; at the time, the working group published results at iptel.org—in September 2002 the code itself was published under the GPL. The first fork of SER came in 2005—OpenSER—which would later merge back  into the code that would become Kamailio. The codebases of SER and OpenSER (by then known as Kamailio) converged in December 2012, and it was decided to continue to use Kamailio as the main name of the project, which remains open source. During the first years of development, "serweb"—a web-based user provisioning—was available.