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I believe you can just drop into any AT&T store and ask for a prepaid SIM, it'll cost you some money, don't be surprised (at least here in Toronto I can go to any Rogers store and buy a prepaid SIM).
 
Ehh, that I can't help you there. I don't know anything about American wireless carriers and their rules/regulation about porting numbers. Talk to a rep, and see what they can do for you.

I think you can even start forwarding the calls from your old cellphone number to the new AT&T one. Eventually people will just start dialing your new number hearing the message over and over again.
 
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