It depends on your application.
If you never call another activity nor do you change the orientation, Activity_Pause will be called when only you quit the application.
But I do change orientation and I expect the code to fire. It doesn't. I checked it on a number of my projects. I suspect it is a problem with some library update i have, or haven't done. Is there a list of 1.6 ver libraries somewhere?
How did you check it ?
If you set a breakpoint you won't see anything, it doesn't stop in Activity_Pause.
Put a Log statement in the Activity_Pause routine and check it.