My CINGULAR Blackjack Is Locked And says It Need a Certin sim Card To Work Is There A Code To Unock It?

the sim has become blocked. in order to get it working again, you need to contact at&t customer support, they have the codes to unlock it. if you mess with it too much then you will lock up the phone and possible burn the sim and then the only way to unlock is through a service center. generally you have 3 times to enter in the card number (and its usually 4 zeros as the defualt). if then it locks up and still cannot get in, then you have to call and get the puk code from the provided and then they will reset the pin. If the pin was changed and is not default (like if you put a pcode on the phone) then the zeros wont work.