iPhone 4 Activation Problem

Hey guys,

I have an iPhone 4 that I am unable to activate. I have placed my valid At&t sim card and i get "SIM not valid - The SIM card inserted in this iPhone does not appear to be supported" even though it works when i put it into my iPhone. I have called at&t and ran the imei number and they said it was clean, not tied to anyone's account. I have tried restoring it to no avail. Anyone have any suggestions?

Thanks