SIM Not Valid - iPhone 4S AT&T
I purchased an iPhone 4S off of craigslist and am not able to activate the phone using an AT&T SIM. When I purchased the phone last week the phone was activated and I could go to settings, apps etc. It didnt have a SIM so I couldnt make a call but everything else looked normal. As soon as I got home I updated iOS to 6.1.2 and went through the update process as usual. To my surprise, I got a "SIM Not Valid" error when I tried to activate the phone. I was using an old ATT micro sim that is not subscribed. I thought that this was likely the issue, so I borrowed a friends subscribed SIM from his 4S. I put the sim in my phone and tried to activate it. BAM.... got the Sim Not Valid error. Obviously something is wrong since my friends sim came from a 4S that is ATT subscribed and working.
What should I do from here? Call AT&T? Call Apple?
I checked the serial number from the sim tray and Apple's site says it has coverage till the end of September 2013 so its still under warranty. Is this error something that is hardware related? Or in a worse case, is this phone stolen and cant be activated?
As an aside, I contacted the fellow that sold me the phone and he said that he was traveling for the next few weeks and couldn't help me. I get the feeling that he doesn't want to even try to help.
iPhone 4S, iOS 6.1.2