Asterisk - An open source telephony switching and private branch exchange service for Linux.
BVoIP - BVoIP allows to easily add Cloud & On-Premise VoIP PBX to the product offering without having to re-invent the wheel or add another layer to the daily operations..
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.
GoTo Connect - Jive PBX is a communication solution that offers remote communication services to companies such as video conferencing, text, and chat services.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.