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.
VirtualPBX - VirtualPBX provides hosted VoIP PBX phone services that give small and medium growing businesses a professional, fully automated call answering and routing solution
CSipSimple - CSipSimple is a SIP application for android devices.
Eir SIP Voice - eir SIP Voice is revolutionizing the core communications tool of business that carries both calls and data over just one connection.