To be accurate, it's on the carrier side, not your phone, as you point out, the phone is capable, Att just doesn't let most non branded phones use it. And if you're on prepaid, no phones (branded or not) get VoLTE.
When? The sarcastic side of me is going with "when pigs fly". But it's more like, when ATT realizes they aren't making a profit off branded phones, but are making a profit off service to non branded.
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