Caller iD-Format Fix FW 3.x fixes the format of phone numbers in each country. I had this problem with my first iPhone 3G, brought from the USA, there was no way to show the numbers in xxx xxx xxx format and they looked like (xxx) xx xxxxx, I had to repair it via SHSH. With this modification your problem will be solved automatically.
Only for 3.x firmwares
You can download it free en Cydia.
You need to have done the jailbreak.