Case in point: Blackberry Priv (2015) v. Blackberry Key2 (2018)

Both models are VoLTE compliant. AT&T whitelisted the former supposedly because they once sold/supported this model directly, but told existing users of the latter to replace their phone or GTFO.

I gladly took the GTFO option and switched to T-Mobile...substantially reduced my monthly bill to less than $17/mo per phone and saw improved reception at the office.