You want to enter your personnal unlock code on your LG KF757, follow the instructions bellow

KF757

1. Insert original sim card and turn phone on
2. Press telephone icon at bottom of screen to bring up the phone keypad
3. Enter 2945#*757#
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 KF757 or LG Secret with another provider, you can get your unlock code on unlock-phone-lg.com