1 - Switch ON your Sony Ericsson W20i without any SIM Card,
2 - Press the left arrow key/button once,
3 - Press the button * twice,
4 - Press the left arrow key/button once,
5 - The message "Personalize with MNC2" will appear, (or you will have displayed an unlock menu, select the lock you want to unlock)
6 - Enter the Unlock Code.