Let's just say, most of the known world.
WTI has three cellular modem options, for the United States we have a Verizon cellular modem, for North America (including the U.S., Canada and Mexico) we have the AT&T cellular modem and the third one WTI has is the World cellular modem, that covers the rest of the known world.
Here are the tech specs for:
Verizon Cellular Modem:
AT&T Cellular Modem:
World Cellular Modem: