I have a Project Fi data-only SIM in my iPhone 6 (iOS 10). The data-only SIM allows my GSM-only iPhone (bought with an AT&T SIM) to get service from T-Mobile and would let me get service from Sprint (if my iPhone had CDMA band support.
When I put the Fi SIM card in, the Settings app got a new row called 'Cellular'. In the Cellular menu, there is a single toggle labeled 'Automatic' that defaults to off. If I turn off Automatic Carrier Selection, what will I be able to do? Will my iPhone actually be able to access Sprint? What's going on here, and what's the purpose of the Automatic toggle?
Here's an image:
tl;dr: What does the automatic toggle above control and what will happen if I choose a carrier other than Project Fi?
To clarify: Project Fi is a MVNO carrier run by Google that allows Nexus and Pixel phone owners to connect to T-Mobile, Sprint, and US Cellular networks under one pay-as-you-go plan. I have my main Fi sim in a Nexus 6P. The carrier also offers data-only SIMs to pop into a tablet or spare phone to give a secondary device data service to the same three networks. That's the data-only SIM I'm talking about here.