Detailed analysis of how Project Fi really works (link)

The following link is a very good read about how Project Fi works. Things touched upon:

1. The Google Fi SIM has two profiles
2. The active profile switches between networks
3. Switching networks is fast, but not instantaneous
4. Calls do handover from Wi-Fi to cellular…
5. …but not seamlessly…
6. …and don’t transfer back
7. Calls do not handover between cellular networks
8. Data does not handover between networks
9. The Google Fi SIM does work in other phones
10. Global roaming is done only through T-Mobile

http://nicholasarmstrong.com/2015/08...ver-google-fi/

All credit goes to Nicholas Armstrong.