Asterisk - An open source telephony switching and private branch exchange service for Linux.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
Sipdroid - Free and open-source SIP/VoIP Client