Go phone w/ contract sim?

If my AT&T phone on a contract breaks can I just buy a cheap AT&T prepaid go phone and put my sim card into it and use it or does it need to be unlocked or something?
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Yes, you just put your SIM in the prepaid phone and it works fine. I had that happen to me. My phone broke, I went to AT&T store, sold me prepaid phone, put in SIM card and was calling again.