These are instructions on how to unlock the iPhone 3G and iPhone 3GS for use with any GSM cellphone carrier using UltraSn0w. This will work with the iOS 4.0 firmware.

Before you can follow these instructions you must have a Jailbroken iPhone and you must be on the 04.26.08, 05.11.07, 05.12.01, or 05.13.04 baseband(modem firmware).

To find your firmware and modem firmware(baseband) versions you can follow Link tutorial.

If you are on T-Mobile remember to turn off 3G before starting...

Enter repo666.ultrasn0w.com/ as the source url and press the Add Source button.


Once the source has been added press the large Return to Cydia button.


Press the Done button at the top right of the screen.


Press to select repo666.ultrasn0w.com from the list of user entered sources.


Press to select ultrasn0w from the list of packages


Press the Install button at the top right of the screen


Press the Confirm button at the top right of the screen.


After installation completes successfully press the large Return to Cydia button.


Source:iclarified.com