So you want to unlock you Google Android T-Mobile G1? No problem, it’s easy, in fact anyone can unlock their G1 because Unlock-TmobileG1.com will sell you the unlocking code for $22.99.
Once you have placed an order including your IMEI code you will receive an eight-digit unlock code for your device, Your IMEI number can be found by pressing *#06# in the dialler, or by going into your settings, selecting “About phone” and then selecting “Status”.
Once a user receives their unlock code they insert a non T-Mobile SIM card and the G1 will prompt for the unlock code, enter it and then hit unlock. Once done the user will receive a notification saying “Network Unlock Successful”, and that’s it job done the T-Mobile G1 is now unlocked.
Source — unlock-tmobileg1