It was the screen, but the piezo strain was applied at the mounting to the frame. They really aren't separate. I believe I worked on this design (though not directly on the piezo) and it was a pain. The easiest way to have a nice solid phone that doesn't rattle or come apart is to laminate it together. You can't do that, if the front surface needs to move relative to the back.
You need a large area (~10cm diameter) moving a reasonable distance (~100s um) to make a speaker work at acceptable volumes in the kHz frequency range.
