No, it is, or soon will be, an issue on all U.S. networks and the MVNOs that run on them.
T-mobile has delayed turning off their 3G network completely until later next year, but has probably already started re-purposing their 3G spectrum in select locales. That will affect connectivity for users in those areas, if their phones can’t do Voice over LTE.
AT&T has said they’ll be shutting down their 3G network in February, but I don’t know if that means a hard switch-off, or the beginning of their own re-purposing schedule.
All the networks already are refusing activation for phones that are not certified for VoLTE services on their networks (i.e. certified by OEM/Carrier testing and certification). T-mobile is typically more BYOP- (bring your own phone) friendly than AT&T, which has already published a short list of “approved” devices, mainly the ones that they themselves have sold, I think. Verizon also has a whitelist, from what I understand, but I’ve read a couple of comments on the internet that some users were able to get certain other unofficially approved phones provisioned.
Discussion on VoLTE: VoLTE support progress