Google Fi is virtual mobile carrier network service for Wi-Fi. As with a number of other mobile virtual network operator (MVNO) providers, Google Fi runs off the backbone of other telecom carriers. Google differentiates itself from competitors by offering pay-per-use data plans and international roaming with no extra charges. Google Fi also gives customers credit for unused data and a bill protection service to help customers avoid exceeding data limits.
When Wi-Fi is available, the service will automatically connect to a hotspot through a virtual private network (VPN). Google Fi switches seamlessly between Sprint, T-Mobile, Three and U.S. Cellular networks and automatically transition to a cellular network when Wi-Fi coverage is not available.
As of this writing, Google SIM cards are available on the Fi site for free and can be purchased for $10 at Best Buy. Plans are $20 per month for unlimited phone calls and texts with a $15 charge for each additional phone line.