As I understand it, US carriers already turned off 2G and are turning 3G off real soon now. If your phone doesn't do VoLTE and wants to do a voice call, that won't work when 3G is turned off.
AT&T takes it a step further. Only if my phone does do VoLTE and AT&T puts it on their magic whitelist (which you can bet, for example, is not going to have the PinePhone on it) am I going to be able to use my phone at all on AT&T's network (well, at least that's my understanding). Other carriers might not have a whitelist though—but I'm not sure.
I recall T-Mobile used to be pretty uptight about using band X for LTE only for VoLTE phones because they wouldn't be able to go from data to dialing 911 if they only had band X coverage (it was better for TMo if the user had no coverage in that case for regulatory reasons), but I think TMo is less strict about what phones they'll provision for VoLTE. I only remember trouble getting my first LTE phone to work, but LTE was newish; once I got a TMo firmware on it, it would provision and unlock whatever band it was I was trying to use, even though the IMEI would have been for an ATT model.
