A new method to downgrade iPhone 3G 06.15.00 Baseband to 05.13.xx is available now.It will enable you to restore your device on the official stock firmware using iTunes which was otherwise not possible on iPad’s 06.15.00 baseband.This method currently only works with the iPhone 3G with bootloader 5.8.The procedure is straightforward.It is also highly recommend you to backup everything on your device before following the procedure.
What you will need:
The steps are as follows:
Step 1: Download the required certificate file ICE2-06.15.00.cert from here.
Step 2: Install Fuzzyband from Cydia and start this app.
Step 3: Connect your iPhone 3G with computer via USB cable.
Step 4: Start i-FunBox and click File, then navigate to Applications/Fuzzyband.app, right-click and then select Copy from PC.
Step 5: Select the ICE2-06.15.00.cert file you downloaded earlier and then copy it to this location: Applications/Fuzzyband.app.
Step 6: Now simply disconnect your iPhone from your computer and start Fuzzyband app (which you previously installed using Cydia) on your iPhone.
Step 7: In Fuzzyband, you will now see your current version of Baseband, (which should still be 06.15.00). But now you will have the option to downgrade it to version 05.13.xx.
Once you have downgraded, Go to Settings –> About, and scroll down to check your Baseband which should now read 05.13.xx (which is unlockable via Ultrasn0w). You can now also update or restore any firmware without iTunes throwing errors.