Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
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
Sipdroid - Free and open-source SIP/VoIP Client
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.