Asterisk - Easy to use but advanced IP PBX system, VoIP gateway and conference server. (Source Code) GPL-2.0C/deb
Eqivo - Eqivo implements an API layer on top of FreeSWITCH facilitating integration between web applications and voice/video-enabled endpoints such as traditional phone lines (PSTN), VoIP phones, webRTC clients etc. (Source Code) MITDocker/PHP
Flexisip - Complete, modular and scalable SIP server, includes a push gateway, to deliver SIP incoming calls or text messages on mobile device platforms where push notifications are required to receive information when the app is not active in the foreground. (Source Code) AGPL-3.0C/Docker
Freepbx - Web-based open source GUI that controls and manages Asterisk. (Source Code) GPL-2.0PHP