Asterisk - An open source telephony switching and private branch exchange service for Linux.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
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.
GlobalMeet Collaboration - From cloud PBX to conference calls and meetings to online events, GlobalMeet is a business communications platform that helps take you where you need to go.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs