Pretty much all of the mobile phone carriers require you to explicitly
enable international roaming of any sort on a post-paid account.
Allowing this exposes the carrier to much larger out-of-pocket charges
from their overseas roaming partners than normal domestic service does,
so they want a second chance to do a credit check for customers wanting
this to maximize the probability that you'll pay your bill (the only
exception to this might be Canada, where I suspect most US carriers pay
relatively modest roaming fees).

It is easy to run up a huge bill while roaming internationally whether
the service is voice or data.

Dennis Ferguson