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.
Eir SIP Voice - eir SIP Voice is revolutionizing the core communications tool of business that carries both calls and data over just one connection.
Wazo.io - The UCaaS Platform Built for MSPs
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...