I have been on T-Mobile since about 2014 or so. I do recall making a few test calls when this feature was first included in my plan in 2022, but I believe since then it has no longer worked, so I suspect voice service has been blocked.

I have a flight on Alaska Air in two days, and will try and make a test call on the departing and return flights as a test on their system. I will post both flight results as they occur. Or Shortly Thereafter, If I Don't Get Service, LOL