From BlackBerryFAQ
To determine if your BlackBerry is "unlocked" for use on carriers other than for which it is branded:
1. On your BlackBerry, go to Options > Advanced Options > Sim Card.
2. At that screen type MEPD (see note below) on your keyboard. A new menu will pop up.
- If your BlackBerry has a SureType keyboard (71xx and 81xx devices) you will need to double-tap the P, so the actual keys entered become M E P P D.
3. Look for Network in the list.
4. If your device is "unlocked", it should say Disabled or Inactive. If it says Active, it's still locked to that carrier.
Images of Locked Device
Images of Unlocked Device
