FluffyChat - Open. Nonprofit. Cute ?
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
Molly (Signal fork) - Molly is an independent Signal fork for Android
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).