Samsung SGH-D307

Has anyone gotten the D307 to work off the Cingular Network... I have six active AT&T lines. I have bought 3 D307's from 3 different people and every time I put in an AT&T Sim and turn them on each screen displaied "WRONG CARD" I can put in a non activated Cingular Sim the phone comes up fine (Just No Service). I can put in an active Cingular Sim and everything in the phone works. Each Vendor swears that the phones have been unlocked and will work with my AT&T Sims...
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Discouraged AT&T User ,
Roger