A phone with a pink case is calibrating a faceup, and reading at zero degrees.
The smart phone displays the message Calibrate Faceup on the bottom of the screen.
A smartphone with a purple case and the display screen with the words Calibrate Faceup
A cell phone with a level displayed on it and it says Calibrate Faceup on it.
A level measuring app shows that the surface in question is exactly level.