Asterisk - An open source telephony switching and private branch exchange service for Linux.
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.
3CX Phone System - 3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such...
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Wazo - Wazo is a unified communications platform based on Asterisk and focused on extensibility.
Wazo IP Telephony - Wazo IP Telephony is an open-source platform that is designed to build your own IP telecom platform and provides IP telecopy, mobility.