How do I lock the keypad on my Nokia device? - Nokia FAQ
To lock (or unlock) the keypad manually in the home screen, press first the left selection key and then the right selection key. Or press the power key briefly, and select Lock keypad.
The keypad of your device locks automatically after the keypad autolock period. To change the autolock period go to Menu > Ctrl. panel > Settings > General > Security > Phone and SIM card > Keypad autolock period.
When the keypad is locked, you can still answer incoming calls and make calls the official emergency numbers programmed into your device.
Note: With the Nokia E5-00, the power key is the same key as the End key. When pressing the end key inside an application, it will cause the device to go into the homescreen, instead of the usual options to change the profiles and lock the device.