Asterisk - An open source telephony switching and private branch exchange service for Linux.
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.
3CX Phone System - 3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such...
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
VitalPBX - VitalPBX is a full-feature PBX (phone system) based on Linux and Asterisk.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies