Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Callmama - Get cheap international calling, international virtual number, Call Mama Gives an added advantage to send message and make calls to non-user of Callmama App.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
CallHippo - Easy to use Phone System with World-Class Support
Sipdroid - Free and open-source SIP/VoIP Client
Numero eSIM - Numero eSIM is a phone-number app that makes voice and data possible anywhere in the world, for free.