To avoid accidentally making a call when your phone is in your pocket or bag, lock the keys and screen of your phone.
Slide the key lock switch.
If the key lock switch is out of reach, to unlock the phone, press the scroll key, and select Unlock.
Set the keys and screen to lock automatically
Select > Settings and Phone > Display > Screen/keylock time-out.
Define the length of time after which the keys and screen are locked automatically.