Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
GlobalMeet Collaboration - From cloud PBX to conference calls and meetings to online events, GlobalMeet is a business communications platform that helps take you where you need to go.
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.
MiCloud Connect - Secure and flexible cloud phone services and unified communications.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.