I'm on a pre-paid Virgin Mobile USA plan. There are EVEN a few spots in southern New Jersey (with no signal

)--ouside the US it CAN'T work [part of the plan]
Which is why I said vast majority, there are exceptions, although those exceptions are going to be a much smaller share of international travelers than they are even of the population as a whole (Virgin Mobile, Boost, etc. aren't exactly targeted at the frequent traveler to Europe).