Detect lock/sleep button iphone

How do I detect when the user has pressed the lock/sleep button at the top of the iPhone?
I've tried:
OnMinimize
OnButtonDown
OnButtonUp
OnNativeEvent
But nothing is called when I press that button. I'd like to know so I can pause/resume the game.
I've tried:
OnMinimize
OnButtonDown
OnButtonUp
OnNativeEvent
But nothing is called when I press that button. I'd like to know so I can pause/resume the game.