Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
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.
Zoom - Equip your team with tools designed to collaborate, connect, and engage with teammates and customers, no matter where you’re located, all in one platform.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.