I have an old blackberry curve phone 8300 that is not in service anymore and I?

know someone in Mexico that wants the phone. The phone was under contract with at&t and it was never unlocked. If this person buys a sim card in Mexico and insert it into the blackberry, will the phone work? Or does the phone need to be unlocked to work in Mexico? If so, how do you unlock a phone that's not in service anymore??