If you own a GSM iPhone 4S or iPhone 4 and want to unlock it to use on other carriers such as on T-Mobile USA, then you can unlock your device with a hack thanks to a software bug recently found in iOS 5. You don’t need to make any software or hardware changes to your device and can simply unlock it by following these steps.
First of all make sure you have a GSM iPhone 4S and iPhone 4. Insert your GSM Sim in your iPhone and follow the instructions below. Proceed at your own risk…
- Insert original carrier AT&T SIM card
- Dial 611 for AT&T customer service hotline and drop the call
- Turn on Airplane Mode
- Take out AT&T SIM card
- Insert T-Mobile SIM card
- Make sure WiFi is off ( also tap on ‘Forget this Network” to make sure it doesn’t connect automatically later)
- Switch off Airplane Mode and iPhone will search for network. This is followed by the Apple splash screen appearing.
- Activation Required will be displayed on the screen
- EDGE network will activate automatically – notice the ‘E’ on the top left corner of the screen
- Wait for about 20-30 seconds and turn off the phone
- Turn on iPhone and the same Activation Required screen will be displayed
- When you see one signal bar, tap on Use Cellular Connection
- Eject SIM card
- Activation Required screen will be displayed the second time
- Insert SIM card
- Unlocked!
Many users have found this method useful and have successfully unlocked there devices.
(Source)