How do I get my iPhone 3GS to recognize sim card?

My iPhone 3GS does not recognize any sim card. It does not even show the "No Sim Card Installed" error message. It just keep showing "searching" and then "no service." I get this with both a sim card in and without a sim card. I got AT&T to unlock my iPhone but I cannot complete the unlock process because I cannot activate the phone. I am stuck on the activation screen because it will not recognize any sim card even an active AT&T card. And when I try to activate it using iTunes I just get the "activation server is temporarily unavailable" message. I have try restoring the phone in DFU and recovery mode and everything else under the moon. I am at my wits end. Please HELP!!!