At&t data plan with unbranded Android phones?

I don't have any data plan with At&t and have no intention of signing up. If I were to purchase an oversea smartphone / android phone, will they be able to detect and automatically charge me extra for the data plan? Some of these brands are unheard of. I had an LG smartphone before, which i purchased from Korea, they didnt charge me extra since they didn't recognize it. I'm not sure if I got lucky or if that's how it is for all overseas smartphones.