Are you restoring from DFU mode? I believe that you need to do that. Are you "Hacktivating" it, or do you have an AT&T SIM installed in the phone?
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Are you restoring from DFU mode? I believe that you need to do that when restoring a custom IPSW. Are you "Hacktivating" it, or do you have an AT&T SIM installed in the phone?