Although I don't have the XL I do have the unlocked 950DS bought from Microsoft (U.S.) and I'm using it on T-Mobile. There is no Wi-Fi calling but there is Visual Voicemail (and it works).
Visual Voicemail works but no Wi-Fi calling. I have a Lumia 1520.2 (at&t) and I just switched to T-Mobile in Sep 2016. Running version:1607 - 9030.48 (02540.00019.14484.37000) IMS: not registered (though this means nothing in Windows 10)