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
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
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
Talkatone - Android App on. Google Play. Stop paying for cell minutes, texts & landlines; Turn your iPad or iPod touch into a phone; Call/Text even in areas with bad cell coverage; Call/Text when travelling without paying for expensive international minutes.