Official T-Mobile FAQ:
Can I make or receive calls from outside of the United States?
T-Mobile cannot guarantee Wi-Fi service outside your home or US HotSpot locations.
Calling from international locations with your T-Mobile HotSpot Phone and Wireless Router works the same as if calling from the US. The normal domestic rates apply when making a call to a location within the US, and international long distance applies when making a call to a location outside the US.
NOTE: There are no call hand offs to or from networks other than T-Mobile USAxxx8217;s network, and Wi-Fi calls cannot be made or received at T-Mobile HotSpot locations outside of the US.
Link to it: T-Mobile HotSpot @Home – Technical Information