There were a number of cheap Android devices that had LTE, but never had VoLTE enabled. Instead of allowing them to work as data only devices, most carriers just blacklisted them from their networks.
Yup, got burned by this. I had a POCO X3 (4G LTE but no VoLTE) which I'd still be using today had AT&T not labeled it as a 3G phone. Even just attempting to use it with my SIM card now automatically deactivates my plan.
Now they have a phone whitelist restricted to (mostly) mainstream brands...
