Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Stork.ai - AI-Based Collaboration for Hybrid Teams. Rich media messaging, advanced video and voice conferencing, auto-transcribing and screen sharing. Serendipitous meetings at the water-cooler with your human and AI teammates.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Ring4 Meeting - Dead simple video conferencing - free, in browser
Sipdroid - Free and open-source SIP/VoIP Client
Suttna - Keep every team member in the loop