Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
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.
Sipdroid - Free and open-source SIP/VoIP Client
Molly (Signal fork) - Molly is an independent Signal fork for Android