Asterisk - An open source telephony switching and private branch exchange service for Linux.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
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.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Wazo.io - The UCaaS Platform Built for MSPs
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.