Asterisk - An open source telephony switching and private branch exchange service for Linux.
FileFlex - FileFlex: Access Share Stream developed and published by Qnext.
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.
Zoho Meeting - Free video conferencing, online meeting, desktop sharing solution from Zoho.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Voxox CloudPhone - Get a local or toll-free business number that automatically answers, routes and transfers calls, plus other professional business phone services.