Unlock Key M,ount And Blade How Does One Lock And Unlock The Key Pad Of Nokia N72?

How does one lock and unlock the key pad of Nokia N72? - unlock key m,ount and blade

When I did not use the phone during working hours, I hold in my pocket, but I do not know how to lock the keypad to press the keys and select a "no" by mistake. I know how I can lock combination (numeric keypad as it used to lock the phone when not using, and then, as I unlock it), please help.