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
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
TextMe - TextMe is a free calls and messaging application at both local and international levels.
Sipdroid - Free and open-source SIP/VoIP Client
FluffyChat - Open. Nonprofit. Cute ?