Ooma - Ooma us a free internet based phone service the delivers the efficient VoIP services at clear calls.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Zaplee - Zaplee is a global cloud-based PBX phone system.
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.
Grasshopper - Grasshopper enables entrepreneurs to build a virtual office that connects callers to employees, wherever they are.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.