Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
MicroSIP - MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack.
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
TalkU - TalkU is one of the best calling and messaging applications at the moment available over the internet for getting in touch with those who matters you most.