Help! Bought a 3GS to use as an iPod and stuck at activation!

So I bought my daughter a used 3GS to use as an iPod. As soon as I got home I restored and upgraded to iOS 6 (the seller hadn't wiped any of their personal data). The problem is that after upgrading it's trying to activate and does not succeed so the phone is stuck at the activation screen. I've tried activating on the phone and in iTunes. I tried activating with the inactive T-Mobile SIM that came with it and an active AT&T SIM from another phone. Neither works.

So, with Christmas just around the corner, how can I get this phone activated (or get around the activation screen), so my daughter can get the iPod she's been wanting?

TIA!