Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Sipdroid - Free and open-source SIP/VoIP Client
MizuDroid - MizuDroid is an app that allows you to make voice and video calls over VoIP protocol.
Fongo - free VoIP app - free Canadian phone number
Jitsi - Open-source video calling and chat platform.