How can I downgrade my iPhone 3G baseband to 02.28.00?
If you've used Apple's iTunes update process to update your iPhone 3G to OS 2.2.1, you may have noticed the baseband was also updated from 02.28.00 to 02.30.03. Unlocking a jailbroken iPhone with yellowsn0w to use it on additional wireless carriers requires baseband version 02.28.00.
The solution is now available on Cydia: 3G Fuzzyband Downgrader. This software will safely downgrade any newer baseband to 02.28.00.
Warning: if your iPhone 3G was manufactured after September 2008, it's possible your bootloader version is higher than 5.08, and this software will not downgrade your baseband.
After you've jailbroken your iPhone 2.2.1 with QuickPwn or PwnageTool, download and install 3G Fuzzyband Downgrader using Cydia.When you run 3G Fuzzyband Downgrader, it will check your iPhone's bootloader version automatically. The software will not work on iPhones with bootloader versions higher than 5.08. The program will say "Not Supported" and you are stuck with your current baseband.
If the bootloader version checks out, 3G Fuzzyband Downgrader will say "Ready to Downgrade!" Just touch the white button and the software will apply the patches needed to downgrade the baseband.
If the modification is successful, the message "I HAZ DOWNGRADE!" will appear.
Now you can install yellowsn0w and use your iPhone 3G with OS 2.2.1 on another GSM carrier.