Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Mitel - Business Communications and Collaboration
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
CSipSimple - CSipSimple is a SIP application for android devices.
CounterPath Softphone Client Suite - CounterPath enables users to make VoIP audio and video calls, send instant messages and manage their presence in an easy-to-use software.