Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FluffyChat - Open. Nonprofit. Cute ?
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Olvid - The most secure messaging app in the world.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Molly (Signal fork) - Molly is an independent Signal fork for Android