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
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
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.
Sipdroid - Free and open-source SIP/VoIP Client
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.