CSipSimple - CSipSimple is a SIP application for android devices.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
HoduPBX - Communication with your team streamlined. Simple & easy!
Mitel - Business Communications and Collaboration
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
Lumicall - Free calls using open technology on Android.