Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
AVOXI - AVOXI to deliver amazing call quality & an exceptional customer service experience.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Sonetel - Sonetel is a communication app that lets you make an international business call at the cost of local calls.
Sipdroid - Free and open-source SIP/VoIP Client
Global Call Forwarding - Global Call Forwarding offers local and international toll free numbers which can be forwarded to any phone, anywhere in the world.