To lock your phone use either of the following processes.

Press and hold the (*) key from the home screen to lock the keyboard.
  -- OR --
Scroll to the menu icon that locks the keyboard.

Once locked then you can't use the scroller or the roller ball to unlock it so the following should unlock it.

Press the (*) key and the SEND key (green answer button) to unlock the keyboard.

If there are continued problems and you can't unlock it then give the Help Desk a call.