iPhone 3G - How do you unlock and activate without original sim?

I have an iPhone 3G and its an old phone that i want to use as an iPod and we don't have anything to do with the original carrier anymore - no old sim cards or anything from then lying around, . The phone gained all its apps from an old friends computer and itunes account so in order to put music on it , i backed it up and I restored it and now it won't let me into the phone because it needs a valid sim with no lock on it - which we don't have , ive tried my sim on my current phone and its not working and itunes keeps showing the message 'We're sorry, we are unable to continue with your activation at this time. Please try again later, or contact customer care.' when synced with the phone. Please help?