Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Ooma Office - VoIP-based business phone system solution for small firms
Mitel - Business Communications and Collaboration
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
Vonage Business - Business phone system.
CSipSimple - CSipSimple is a SIP application for android devices.