I've just bought an iPhone 3G 8G. However the seller has upgrade OS to 3.1.3 so that the firmware is 05.12.01 now. I could downgrade to 3.1.2 and jailbreak it. However, I could not downgrade firmware to 05.11.xx. Now my iPhone can access Internet but making phone call. There always be "No service" message at the network bars. If I change my plan to AT&T, will my iPhone work with AT&T even it was unlocked unsuccessfully? What should I do to make it work with AT&T. Thank you.