They claim to me that they can’t activate a phone on both networks at once since the IMEI ID cannot be registered with Fi and Sprint at same time. Only way around is to switch Fi plan to another phone every time you want to switch to sprint plan. Or at least that’s what I was told.
This is in regards to the Sprint promo but relevant regardless
C/p from SD
4: My phone is/was on Project Fi (Google). What can i do to make it work on this promo?
AFTER removing your phone from Fi (port out or cancel), you have to call them and ask them to deregister the phone or rather make it available for Sprint postpaid use. Because Sprint is part of the Fi network it looks like the phone is already active on there. Takes 24-48 hours to happen once you open a ticket by calling 1-844-TALK-2FI.
Note: When I called, the CSR did not understand when I asked about "deregistering" my phone. Probably the most efficient way to go about this is to explain that you are trying to switch to Sprint and need to be removed from the Project fi list or whatever. My case had to be elevated to a higher up CSR/tech, and after a brief hold, I was told it was taken care of. I was then able to pass go on the sprint IMEI checker.