Unlocking iPhone 3g/3gs on 3.1.3

1. Download the 3.1.2 custom firmware 3g/3gs
2. Make sure you have the latest version of itunes.
3. Turn your phone off.
4. Bring it into DFU mode. To do so, while your iPhone is switched off hold the home button and connect it to the pc. Let go of the home button as soon as you see a data cable with the iTunes logo.
5. Shift click or option click on the "restore" button and find the custom firmware downloaded in step 1.
6. Sit back and enjoy the show on your iPhone. Now the iphone is jailbroken but not unlocked.
7. Now launch Cydia from your iPhone. You wil require an active internet connection on your phone for cydia to work.
8. Press to select 3G Fuzzyband Downgrader from the list of packages & click on install.
9. After installation is completed successfully return to cydia and press home button and launch Fuzzyband.
10. A message will be displayed if your bootloader is not supported,  If you have the 5.08 bootloader you will be able to downgrade from baseband 05.12.01. Press the Downgrade button to begin downgrading your baseband.
11. Once the downgrade has completed successfully you will be displayed a Done! message saying, “I Haz Downgrade!”
12. Open Cydia,Press to Manage from the tabs at the bottom of the screen.Press to select Sources from the menu that appears.Press the Edit button at the top right of the screen.Press the Add button that appears at the top left of the screen.Enter http://repo666.ultrasn0w.com (that last “0” in ultrasn0w is a zero!) as the source address then press the large Add Source button.
13. Now Search for ‘ultrasn0w’ in cydia and install it.
14. Reboot your iPhone 3G/3GS