You want to enter your personnal unlock code on your LG GT505, follow the instructions bellow

GT505

1. Turn phone on without simcard
2. Press telephone icon at bottom of screen to bring up the phone keypad
3. Enter 2945#*505#
4. A menu will pop up, select sim unlock
5. Enter your unlock code and press ok
6. The phone will then reboot and the unlock is complete

If you need your unlock code to use your LG GT505 with another provider, you can get your unlock code on unlock-phone-lg.com