My recommendation is that you lock your phone as soon as you finish using it. Even if the phone is locked you can still answer an incoming call. The lock will disable the touch screen so that it will not go into the menu and mess up settings, auto-dialing up numbers, deleting numbers, sending sms, etc.
In short - LOCK YOUR PHONE WHEN YOU FINISH USING IT!
To enable the phone lock, simply press the top left hand button and the bottom right hand button (the # key). Unlocking the phone is the same key combination.
Given below is a photo with the key combination to lock your phone: