This happened at two separate AT&T stores. (I went to the second one because the clerk at the first one was alone in the store and too busy to deal with my follow-ups.)
Me: I want to know about using my GoPhone [AT&T's cheap prepaid service] in other countries.
Clerk: It doesn't work in other countries.
Me: Really? I've used it in Canada before.
Clark: Oh, it works in Canada.
Me: Perhaps you don't know this, but Canada is another country.
I wasn't laying a gotcha, either: I was conflating two questions. 1) confirm it would work in Canada (when I used it before, I was close to the US border; this time I won't be); and 2) could I use it overseas? Turned out you can, you just have to change the payment plan.
Other option, I suppose, is to buy a disposable phone when I get there, but I don't know anything about those, and Googling "disposable phone" got me a raft of what were intended as helpful articles that were of no help whatever. Either they were answering different questions from the ones I was asking, or else they assumed things I guess I didn't know.