What are some alternatives?
When comparing Eir SIP Voice and Google Voice, you can also consider the following products
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
Viber - Free and secure calls and messages to anyone, anywhere. Keep your conversations going no matter where you are.
Loop Communications - Loop Communications provides hosted VoIP business phone systems to small businesses and mid-sized companies.
UberConference - UberConference provides a rich visual interface for conference calls. Simple, visual, and free to use, it's audio conferencing done right.