Shop Talk

Someone I know purchased an AT&T V600 on e-bay and wants to use it on Cingular's network. I changed technology for him and inserted his sim card and it said to insert sim, then it said contact service provider. We tried Motorola's customer service but there was no help. I would appreciate any suggestions as to what to do now. I was under the impression that all gsm motorola's are unlocked. Does that apply to Cingular GSM Motorola phones only? Thanks in advance for your help.
Tim