FreeSWITCH is a scalable open source cross-platform telephony
platform designed to route and interconnect popular communication
protocols using audio, video, text or any other form of media. It
was created in 2006 to fill the void left by proprietary commercial
solutions. FreeSWITCH also provides a stable telephony platform
on which many applications can be developed using a wide range of
free tools.

WWW: https://freeswitch.org