freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
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.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
VitalPBX - VitalPBX is a full-feature PBX (phone system) based on Linux and Asterisk.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.