Asterisk - An open source telephony switching and private branch exchange service for Linux.

Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized companies.

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.

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..

Kamailio - Kamailio® (successor of former OpenSER and SER) is an open source SIP server released under...

OnSIP - OnSIP offers cloud phone sytem for business communications across all devices.