It's complicated, but the simple (most likely) answer is you won't. I use it all the time, BUT my plan has unlimited calls and texts. That's where it might vary: if your plan charges you for calls, then you will get charged.
WiFi calling basically makes the phone work just as if you were in your home region using T-Mobile's network: whatever costs you there, will cost you over WiFi Calling (long distance calls if not included, etc.) BUT if you use WiFi calling to receive calls and to make calls to numbers you don't usually get charged for, then you won't be charged for those calls either.
Now this is where it gets tricky: some hotels are starting to block WiFi calling on their networks, so you might not be able to use it depending on the hotel or network you're trying to use. Same thing with VPN; some countries actively block them (like China), but in order to deter streaming and clogging up their network, so do some hotels.
VPN wouldn't work to make the phone 'think' you're in the US for calls, however, unless you're using WiFi calling for that. VPN would work on apps that, again, aren't actively blocking use of VPN traffic (like Netflix or DirecTV Now).