Keypad lock (keyguard)
To lock the keypad in the home screen, press the left selection key and * within 1.5 seconds.
To set the device to automatically lock the keypad after a certain time, select 
> Settings and General > Security > Phone and SIM card > Keypad autolock period > User defined , and select the desired time.
To unlock the keypad, select Unlock, and press *. When the device or keypad is locked, calls may be possible to the official emergency number programmed into your device.