Hello there. I had my jailbroken+unlocked iPhone 3G with iOS 4.0 and recently downgraded to 3.1.3. This broke the jailbreak and the unlock but also de-activated the phone so I tried almost everything to make it work again but nothing has worked so far. I bought this phone from a guy in the States and I don't remember how I activated it (I guess using a custom firmware). Anyway, I recently bought an AT&T prepaid SIM card from ebay, inserted it, and when I connect the phone to iTunes, it tells me that the SIM is not compatible. I also have tried to use a custom firmware but I always get the 1600 error. Tried to get to DFU mode using latest Pwnage Tool and it tells me that it could not get into DFU mode, even if I have done this a lot of times. If I get the phone into DFU mode manually, I get the same 1600 error. I really don't know what else can I do right now. Could you help me, please? Thank you very much.