Project Fi is an excellently priced and serviced three LTE carrier + Wifi calling service from Google. Project 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 Project Fi phones only use UMA if no cellular signal is available. Overall I strongly recommend Project Fi for those with officially compatible phones.
Project Fi phones will sit on the preferred carrier in your area. In much of the US, Project Fi phones prefer Sprint. If Sprint has a very poor signal (say you walk into the basement), Project Fi switches to the next strongest carrier, typically T-Mobile. In rural areas of the US where Sprint and T-Mobile are weak, US Cellular may be used. As of Fall 2016, US Cellular is mostly 3G, with 4G in a limited but growing footprint in rural America.
Project Fi may sit on the preferred carrier (e.g. Sprint) even if say T-Mobile has a much stronger signal. It’s only when the preferred carrier signal is lost that Project Fi “roams” to the less preferred carrier. You can search the internet to find dialer codes that force a particular carrier for the duration of a phone call.
Project Fi publishes a map that is roughly a superposition of their host carriers. I compliment Project Fi coverage map for not exaggerating their coverage like AT&T and many other carriers do. In most of the world, Project Fi uses open WiFi where available.
Project 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.
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 Project Fi. You can tell if you’re on a VoLTE call by that the phone remains in LTE mode during a call instead of falling back to 3G. Limiting factors to the Project Fi VoLTE rollout include that Sprint is still working on VoLTE and US Cellular is still working on 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 Project Fi call to get VoLTE.
You can tell if your Project 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 very many LTE bands for generation-leading worldwide LTE coverage.
The Project 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-Project Fi phone, the Project Fi SIM will default to T-Mobile. Some LTE Wear OS smartwatches work with Project Fi.