Asterisk - An open source telephony switching and private branch exchange service for Linux.
Wazo.io - The UCaaS Platform Built for MSPs
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
miniSIPServer - Social & Communications and Development