Apple iPhone 3G

I purchased a phone on E-bay as a backup. It had been restored and had the USB/Itunes prompt, but no emergency call prompt. I restored it and jailbroke it using RedSnow and unlocked it using UltraSnow. When I put in my SIM from my other 3G Iphone, it wouldn't recognize it. The SIM is valid and is a AT&T 3G SIM card that I use in my other phone every day. I am wondering why the phone won't recognize a SIM card. Should I try restoring again? Any ideas? Thanks