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.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Asterisk - An open source telephony switching and private branch exchange service for Linux.
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