Asterisk - An open source telephony switching and private branch exchange service for Linux.
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
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
Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized companies.