I bought my friend's broken iPhone 4 from him after he got a 4S. The only problem with it was a broken dock connector, which I replaced using this iFixit guide. Since I replaced that part, everything works fine except the cellular connection. I did a complete restore of the phone but still nothing. Can anyone tell me what I might have done wrong during the process that would have messed up the cell connection but nothing else? Thanks in advance for your help.