Google Fi technical aspects

Google Fi is an excellently priced and serviced three LTE carrier + Wifi calling service from Google. Google Fi requires specific phones for full 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. Overall we recommend considering Google Fi for those with officially compatible phones.

Carrier switching

Google Fi phones will sit on the preferred carrier in your area. In much of the US, Google Fi phones prefer Sprint. If Sprint 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 where Sprint and T-Mobile are weak, US Cellular coverage may be used. In early 2019, US Cellular has substantial 4G coverage, but large 3G areas remain.

Google Fi may sit on the preferred carrier (e.g. Sprint) even if say T-Mobile has a much stronger signal. When the preferred carrier signal is lost, Google Fi “roams” to the less preferred carrier. Search the internet to find dialer codes that force a particular carrier for the duration of a phone call.

Coverage maps

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:

  1. If any Fi cellular connection available, use cellular (even if Wifi also available)
  2. If no Fi cellular and connected to WEP/WPA WiFi, use UMA VoIP calling
  3. 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.

I think UMA/VoIP calling has the lowest priority since the voice call robustness is lower with UMA than with cellular. I used to use a tablet with Google Hangouts Dialer as my main phone, and eventually the often great, sometimes bad WiFi call quality made me get a regular cell phone.


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. Limiting factors to the Google Fi VoLTE rollout include that Sprint and US Cellular are still working to deploy VoLTE. On the other hand, T-Mobile VoLTE has been working fine for over a year, so at this time you need to be in a T-Mobile Google Fi call with a supported device to get VoLTE.

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 Phone has many LTE bands for generation-leading worldwide LTE coverage.

Non-official phones/watches

The Google Fi SIM cards require a phone with appropriate software to be able to switch between the three LTE carriers (currently T-Mobile, Sprint and US Cellular) and VPN-protected Wifi. If you have a non-Google Fi phone, the Google Fi SIM will default to T-Mobile. Some LTE Wear OS smartwatches work with Google Fi.