Voice on Nokia 225 no longer working.

I have the same issue with my beloved Nokia 225. It can send and receive SMS just fine including multimedia. When I try to dial, it just says "Dialling" indefinitely and then eventually stops dialling. It is not possible to receive incoming calls. Like a previous user, I obtained a new unit of the same model and the behavior is unchanged. I have full bars, and the phone says "LTE" over the bars. I spent an hour on the phone with T-Mobile a week or so ago: they verified that the IMEI indicates a compatible unit, they found no issues with the network, and their standard tests turned up nothing. I brought my phone to a T-Mobile store and the rep's there verified that swapping my SIM card into another unit allows that phone full functionality. I'm quite disappointed at the prospect of having to purchase another phone when this one is 4G, and I've lost faith in T-Mobile's compatibility checks since (as I also verified) their compatibility checker explicitly indicates based on the IMEI that this phone should work.