Comment by Gigachad
From what I remember the issue was that many models of phone would use 4g/5g for everything but emergency calls, which was done over 3G. So the government made the choice to block those phones from the network entirely rather than leave them seemingly working but unable to make emergency calls.
Pretty much, but the govt didn't do any blocking directly. They just told the carriers, "Hey, you must not allow people to use devices on your networks that are unable to make emergency calls, or we'll apply serious penalties."
The carriers then responded, "I notice that there is no requirement that we allow any device that can make emergency calls. So we will only allow devices we also sell (and maybe a few other models, if they're popular enough that we can't get away with not allowing them). And if that means more people than necessary will have to buy new phones, we will happily sell them new phones."