Lock or unlock the keys and screen
To avoid accidentally making a call when your device is in your pocket or bag, lock the keys and screen of your device.
Lock the keys and screen 
Press the power key 
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Unlock the keys and screen
Press the power key or the menu key, and select Unlock.
Set the keys and screen autolock delay
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Select Menu > Settings and Phone > Display > Screen/keylock time-out .
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Define how long your device waits before the screen and keys lock automatically.