It's a combination of a technical requirement, and only officially supporting that required feature in certain firmwares. I have a phone model that is slightly

It's a combination of a technical requirement, and only officially supporting that required feature in certain firmwares. I have a phone model that is slightly different from the one on the approved list but it works OK. Not everyone got so lucky. My Galaxy Note 8 supports VoLTE, but AT&T won't provision it and I don't really know why. My guess was that it would take extra work and they doesn't feel like doing it.