This is what I have, from my research 1-2 years ago:
T-Mobile Unofficially Speaks To Us About Band 12 LTE / VoLTE And Unlocked Smartphones, And Yep It’s Still Confusing
Why Is T-Mobile Telling Manufacturers To Remove Band 12 LTE Support From Some Unlocked Smartphones?
T-Mobile and band 12 support in unlocked smartphones put in the spotlight
T-Mobile: All Band 12 LTE devices on our network must support VoLTE, E911
These are years old, but should still make good reading. In this context, the “band 12” issue is inconsequential, but note the article comments including those from the author. The person who collected the links speculates that T-Mobile’s certification program is no more.
Keep in mind that of the three national US carriers, T-Mobile is/was the most generous. At the time, AT&T operated a strict white list, the approved VoLTE phones consisting of no more than a few dozen phones, most of which they sold themselves. Verizon also used a white list, which wasn’t much better. Again, much of this could have changed. Sadly I don’t have time anymore to keep up the research and advocacy.