Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
XO Hosted PBX - Hosted PBX is a scalable UC solution that uses cloud technology to integrate HD voice & data services, mobility, along with advanced calling
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Sipdroid - Free and open-source SIP/VoIP Client
Sipnetic - Sipnetic is a free VoIP softphone based on the SIP protocol.
MizuDroid - MizuDroid is an app that allows you to make voice and video calls over VoIP protocol.