Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
GlobalMeet Collaboration - From cloud PBX to conference calls and meetings to online events, GlobalMeet is a business communications platform that helps take you where you need to go.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
Sipdroid - Free and open-source SIP/VoIP Client