Have you Jail broken your iPhone on 3.1.2 firmware with Pawnage tool ?
If yes, you might be facing the problem of carrier logo missing from the top left corner is missing. This problem appears to be some some bug in the jail break. This problem can be easily fixed by a Installing a tool called “Carrier Logo Fixer” By BigBoss from cydia repository.
Just go to Cydia and search for Carrier Logo Fixer, Install this tool. Restart the iPhone and carrier logo will be restored. If you face any problem, please leave a comment below.