> There also seems to be an additional barrier of US carriers imminently requiring VoLTE, and in some cases only allowlisting a small number of models.
Is this all carriers, or just some? I know that AT&T is the main one since I'm affected by it (their whitelist arbitrarily removes my OnePlus 6 from being supported this February despite the phone supporting VoLTE), but I heard that other carriers such as T-Mobile are still working without VoLTE. Either way, the whole "requiring VoLTE" thing could even be a problem for some custom ROMs (not even GNU/Linux phones) since I believe VoLTE support is something that ROMs sometimes miss, which will be pretty bad. While I'm not a full "privacy" user (yet), I use custom ROMs for extending the lifetime of my phone and getting newer updates than I would on the stock ROM, and carrier limitations could seriously threaten a lot of things.
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.
I want to say T-Mobile is requiring VoLTE (I recall discussion about this because it effectively mean the Nokia N900 cannot be used on T-Mobile or its NVMOs anymore).
I will comment that the Pinephone works with VoLTE on many carriers (I know it works with T-Mobile), and Purism is actively working on VoLTE for the Librem 5 (I remember seeing them actively working it a few months ago, so I guess they have it working now?).
Is this all carriers, or just some? I know that AT&T is the main one since I'm affected by it (their whitelist arbitrarily removes my OnePlus 6 from being supported this February despite the phone supporting VoLTE), but I heard that other carriers such as T-Mobile are still working without VoLTE. Either way, the whole "requiring VoLTE" thing could even be a problem for some custom ROMs (not even GNU/Linux phones) since I believe VoLTE support is something that ROMs sometimes miss, which will be pretty bad. While I'm not a full "privacy" user (yet), I use custom ROMs for extending the lifetime of my phone and getting newer updates than I would on the stock ROM, and carrier limitations could seriously threaten a lot of things.