Many US carriers put a lock on their phones IMEI number which they issue which restricts the phone use to that netork. IE
if i have an ATT Phone i can only use it on the ATT Network. However, an unlocked phone (Most Cingular phones come unlocked unlike ATT and Tmobile) can be used on any network just by switching the SIM Card (also internationally)
MNeo: Where did you go/call to get it done for $20 at ATT? I just went to an ATT store and they wouldnt do it for me.