Mitel - Business Communications and Collaboration
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
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
Sipdroid - Free and open-source SIP/VoIP Client