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.