Asterisk - An open source telephony switching and private branch exchange service for Linux.
Mitel - Business Communications and Collaboration
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.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Wazo.io - The UCaaS Platform Built for MSPs
XO Hosted PBX - Hosted PBX is a scalable UC solution that uses cloud technology to integrate HD voice & data services, mobility, along with advanced calling