[fury]

Is it a minute overage or a message overage?

If it's a message overage, you get charged this by sending to or receiving from anyone who's not in the U.S.

If it's a minute overage, ask them if it's possible somebody international called and rang your phone, at which point it could be billed as an overage.

They're stupidly over-general about these things on the billing site, and it's really rare to find anyone at AT&T who knows what they're doing to find out the real details.