re:
Wifi calling is not even the whole issue - "unable to use mobile data" is listed too.
But yeah, this would be locked to the Tmo network. If buying a phone intending to use it on another network, you need the US997 unlocked model.
T-mobile will unlock devices for you, but only under certain conditions (most notably by having already paid them sufficiently to use it on their network), if you're a current customer. If you bought the phone from a T-mobile customer and they meet the requirements, they can try to request it to be unlocked.
T-mobile might unlock it for you for a fee but I have no idea how you would request this as a non-customer. There are also 3rd party services that will do this for a fee ($10-25), and this is probably going to be your easiest route, but be careful if you decide to do this that you don't get scammed.