Note: NO SIM is needed to unlock the phone. After phone is unlocked, remove the battery, insert SIM as desired and re-insert the battery. Phone MUST be restarted to use the new SIM card.
1) Power on
2) Go to Settings
3) Go to Options
4) Go to Advanced Options
5) Go to Sim Card
6) HOLD ALT and press: mepd, and then after the screen changed press mep2 (Please note that you will NOT see what you type on the screen) *
7) It will ask you for the unlock code. Enter it and then press trackball to confirm and your phone is unlocked!
You can call your service provider for the unlock code for your 8520 Gemini / Curve. Usually you must be customer for at least six months.
You can also get your unlock code from trusted sites such http://cellunlocker.net/blog/2010/06/08/how-to-unlock-blackberry-8520-gemini-curve/