Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
FluffyChat - Open. Nonprofit. Cute ?
Sipdroid - Free and open-source SIP/VoIP Client
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Molly (Signal fork) - Molly is an independent Signal fork for Android