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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.

It's a combination of money

I think it’s a leftover from the old way where phones were carrier specific. While we had SIM cards, they had some kind of built-in chip that was specific to their carrier.

The phone likely uses 3g to do voice calls and AT&T ended their support for 3G earlier this year.