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.
PBX in a Flash - PBX in a Flash is a PBX system based on Asterisk, FreePBX and CentOS, coming as an ISO download.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Elastix - Elastix is an open source Unified Communications Server software that brings together IP PBX...
Kamailio - Kamailio® (successor of former OpenSER and SER) is an open source SIP server released under...