pH (Pedro Henrique Franceschi) created a
tool to help you downgrade your baseband from 2.30.03 to 2.28.00. To use this tool, you have to have to have an iPhone with bootloader 5.8. If your iPhone was manufactured before September 2008, you’re good. If you bought your iPhone after September 2008 with firmware 2.2.1 out of the box, then you can’t use pHaseBanDowngrader because your bootloader is 5.9.
Here are the instructions on how to downgrade using pHaseBanDowngrader:
1. Download this file
2. SSH into your iPhone and put the downloaded folder (phasebandowngrader) inside the folder /Applications/
3. In MobileTerminal.app, type “login” and enter “root” as user, the default password is “alpine”
4. Now, type “cd /Applications/phasebandowngrader && ./phasebandowngrader”. Follow the screen instructions
5. Done!