What is your intention for needing the "lock icon"?
1) Do you want to lock the keypad to prevent accidental key presses?
2) Do you want to lock the phone to prevent people from accessing your data?
If it is 1), then use the Mute/Lock key found at the top left corner of the phone. All you need to do is toggle it on and off to unlock the keypad. No password needed.
If it is 2), then you need a password to unlock the phone.