iPhone Manual Carrier Selection
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.
Solution 1:
Your phone will not be able to access Sprint or Verizon networks, because it physically lacks the hardware to do so. The GSM radio in your phone cannot communicate with the CDMA signals that Sprint relies on.
Project Fi only works with T-Mobile and Sprint. From the list, we can rule out Verizon, Sprint and 310-830 (Sprint), so our only options are AT&T and T-Mobile. You can try connecting to AT&T if you like, it won't break anything - you'll be informed that you can't register to that network. Therefore, your only option here is T-Mobile (probably represented by Project Fi here). In your case, the Automatic toggle will always connect to T-Mobile because that's the only supported network.
Let's say you could connect to another network on that list, though. Let's say Project Fi supported both T-Mobile and AT&T. Leaving the Automatic toggle on would pick whichever of those networks had a higher signal.
So, to answer your TLDR:
The automatic toggle connects to the network that a) provides the best signal strength, and b) that you're able to use.
It won't work. You'll be given an error, or will receive no signal at all.