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