3CX Phone System - 3CXPhone is a free VoIP phone available for Windows, iPhone or any Android based smart phone such...
PBXware - PBXware is the world’s first and most mature IP PBX Professional Open Standards Turnkey Telephony Platform. Bicom Systems’ softswitch is offered in the Business, Call Center and Multi-Tenant Editions each supporting specific features.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
tSIP - tomeko.net/software: SIP client for Windows.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).