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.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Asterisk - An open source telephony switching and private branch exchange service for Linux.
The Incredible PBX - Incredible PBX is a secure and feature-rich implementation of the Asterisk® PBX.
Wazo - Wazo is a unified communications platform based on Asterisk and focused on extensibility.
VitalPBX - VitalPBX is a full-feature PBX (phone system) based on Linux and Asterisk.