Asterisk - An open source telephony switching and private branch exchange service for Linux.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
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.
Mitel - Business Communications and Collaboration
Kamailio - Kamailio® (successor of former OpenSER and SER) is an open source SIP server released under...
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.