Taking over old iPhone 4, updated to iOS 6, but can't activate? I have borrowed/taken-over my girlfriend's iPhone 4 that had iOS 4.x installed. She has changed phone carrier, so I don't have any active iPhone SIM card at present. 

Anyhow, I updated to iOS 6 and gave it a Wi-Fi connection for its configuration. However, it just states I should contact customer care, but I don't really believe I should need activation? The phone should not be locked as far as I know, but is that the problem? Or is it the change in iTunes account? 

I am not sure how to progress from here... So I am hoping someone here recognizes the problem, so I can explain the problem properly over phone and/or contact the right ones
All you need is a SIM card in the phone while you activiate it first time after upgrade. (Not sure if sim card needs to be same carrier, still active etc. I was lucky I could borrow a friend's SIM card that incidently was also same carrier as the phone had been on earlier)