My friend got a 3G and gave me her old 2G. I have at&t so I updated the phone, put in my SIM and activated it. iPhone worked fine for a whole day and then when I was charging it, the phone frooze. It turned of, turned on and came up with a message telling me to restore the iPhone. I connected it to iTunes, but it won't restore because it says my SIM is invalid. Is there anything I can do???