Google Fi is reasonably priced (but not the cheapest), using three LTE carrier + WiFi UMA calling service from Google. Google Fi requires specific phones for full network-switching functionality. UMA WiFi calling as with every carrier does not always have flawless performance. This is not a big issue since Google Fi phones only use UMA if no cellular signal is available. Google Fi is a better value for those with officially compatible network-switching phones.
Google Fi phones will typically sit on the preferred carrier in your area. If signal is lost (e.g. walking into a building) Google Fi switches to the next strongest carrier, which is typically T-Mobile.
In rural US areas US Cellular coverage may be used. US Cellular has a mix of 4G and 3G coverage.
Google Fi may sit on the preferred carrier even if another carrier has a much stronger signal. When the preferred carrier signal is lost, Google Fi “roams” to the less preferred carrier. Dialer codes can force Fi phones to a particular carrier for the duration of a phone call.
Google Fi publishes a map that is roughly a superposition of their host carriers. I compliment Google Fi coverage map for not exaggerating LTE coverage area like many other carriers do. In most of the world, Google Fi uses open WiFi where available.
Google Fi currently behaves with the following descending priority order for VOICE calls:
- If any Fi cellular connection available, use cellular (even if Wifi also available)
- If no Fi cellular and connected to WEP/WPA WiFi, use UMA VoIP calling
- If no Fi cellular and not connected to WiFi but open good WiFi available, use UMA VoIP over VPN. You’ll see a lock symbol in the status bar.
VoLTE is being tested on Google Fi. You can tell if you’re on a VoLTE call by the phone remaining in LTE mode during a call instead of falling back to 3G. T-Mobile has deployed VoLTE and so has US Cellular.
You can tell if your Google Fi phone is included in the test by seeing if you have under Settings → Cellular Networks the option for “Enhanced 4G LTE Mode”. If not, you’re not in the VoLTE test yet.
Outside the VoLTE test sites, your phone will fall back to a 2G or 3G connection to make a voice call. This is a disadvantage many phones on many carriers suffer, as often the best coverage by a factor of 10 dB or so is on the 700 / 800 MHz LTE band, yet the phone gets forced to a weak 1.8 / 1.9 GHz signal for voice calls. Many times, the phone will have an 850 MHz 2G / 3G connection available, so the disadvantage is not so large for voice calls vs. LTE.
The Google Pixel phones have many LTE bands for generation-leading worldwide LTE coverage.
Google Fi requires a phone with appropriate software to be able to switch between the LTE carriers and VPN-protected Wifi. If you have a non-Google Fi phone, Google Fi defaults to T-Mobile. Some LTE Wear OS smartwatches work with Google Fi.