T-Mobile was allocated 3G frequencies in the 1700 MHz band, which is used nowhere else in the world for 3G or cell phone service of any kind. Just about everyone else in the world uses 2100 MHz for 3G service.

This is in addition to the existing cellular bands, which are different in North America than most other places. North America (and some parts of Latin America) uses 850 MHz and 1900 MHz for cell phone service while the rest of the world uses 900 MHz and 1800 MHz.