Almost all of iPhone 3G and iPhone 3GS users upgraded their baseband to iPad’s 1 which is 06.15.00 to be able to unlock it. The following method will allow you to downgrade the baseband of apple iphone 3G from 06.15.00 to 5.13.xx and it works only on iPhone 3G bootloader 5.8 which is unlockable using Ultrasn0w.  Downgrading process 06.15.00 baseband to 05.13xx on iPhone 3G must have Jailbroken iPhone 3G, i-FunBox (for Windows) or DiskAid (for Mac), Fuzzyband installed via Cydia and Certificate to use with Fuzzyband. Follow this method below to downgrade 06.15.00 baseband to 05.13xx:

1. Download certificate file ICE2-06.15.00.cert from here.
2. Install Fuzzyband from Cydia and start this app.
3. Connect iPhone 3G to you pc through USB cable.
4. Initiate i-FunBox. Click on File, then browse to Applications/Fuzzyband.app. right-click and then select Copy from PC.
5. Select the ICE2-06.15.00.cert file you already downloaded and copy to the following  location: Applications/Fuzzyband.app.
6. Disconnect your iPhone from your pc and start Fuzzyband app on your iPhone 3G.
7. In Fuzzyband application, you can see your current Baseband version is 06.15.00 and you have the option to downgrade it to version 05.13.xx.
8. Once you have downgraded your iPhone, go to settings  optionin your iPhone and select About. Scroll down to check your Baseband which should now read 05.13.xx (which is unlockable via Ultrasn0w).