Wow, carriers can just remotely lock an unlocked phone? How does this even work? Is this some kind of command that the SIM card sends the phone that the phone recognizes?
The blame here lies on Best Buy (for not making it clear that this occurs) and Apple (for designing and supporting "flex activation" -- which will still carrier-lock the phone even if a MVNO SIM, such as a Fi SIM, is inserted).
The locking is already there, but the operator is chosen at the activation so that Best Buy can sell one model for any operator.
So the blame is actually Apple’s for blindly allowing this behavior, Best Buy’s for not advertising it, T-Mobile’s for allowing this through MNVO’s, and Google’s for not knowing that this happens and not allowing unlocks.
And they said my Node dependency tree was a mess!
If you buy it subsidized or through a carrier payment plan, this works as intended (and they'll refuse to let you have the phone before the SIM is inserted). If you meant to buy an unlocked phone, you've been given an annoying lesson to buy directly from the manufacturer.
And now I also have to wonder: do eSIMs have the same vulnerability to bad-actor carriers? Is this a risk with any of the companies Airalo has partnered with?
Apple’s SIM lock “feature” checks in with Apple over the internet (including over the disallowed SIM, since the modem itself is always unlocked) and refuses to let you proceed if the current SIM doesn’t match the policy returned by the backend.
Obviously the policy can be changed at any time. This is also how this “flex carrier lock” is implemented.
If it's a Best Buy "unlocked" iPhone it can! It was unlocked when he bought it, but locked onto the first network it was activated (Fi is T-Mobile[1] MVNO)
1. and AT&T too for fully supported Fi phones - you can get seamlessly handed over between T-Mobile, AT&T and wifi. However, iPhone are not fully supported, so they exclusively use T-Mobile.