Find "Change password"
Press Applications.
Press Settings.
Press Call settings.
Press Voice call.
Press Call barring.
Press the Menu key.
Press Change password.
Change barring password
Press Old password and key in your current barring password (default is 0000).
Press New password and key in a new barring password.
Press Cofirm password and key in the new barring password again and press Confirm.
Exit
Press the Home key to return to standby mode.