What matters is, with which carrier does T-Mobile have a roaming agreement?

During setup, your available network selection (Automatic Off) will typically populate with three to as many as six or eight. Generally speaking, the top 2-4 may be roaming partners. As you work down the list it becomes less likely they are. It's easier to just start at the bottom of the list in most cases. An MVNO might identify as the native carrier or as it's resale name. In either case, T-Mobile MIGHT roam with either or neither. It only takes a few minutes to ID one that doesn't roam, and that's all you need for that location. You should verify in the Status Bar at the top of your screen your voice carrier is "Using Cellular Data" frequently. I do this prior to any call, and out of habit any time I'm using my phone. You eventually get pretty comfortable with the IMS feature. It's also great for calling between travel companions with the same service, without fees.