Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Asterisk - An open source telephony switching and private branch exchange service for Linux.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
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.
We-Talk - Free online chat rooms where people chat and meet each other and chat with the stranger and learn new cultures and make friends.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.