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.
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.