Facebook messenger like fullscreen activiy

  android, android-phone-call, call, notifications

I would like to know haw facebook messenger can show call activity on top of all opened apps. Call activity interrupt any app even if user is touching or typing on front app.
I do some investigation it is not a high important notification with full screen intent. Because in this case phone will choose to open full screen intent or heads up notification. This not the case facebook use default notification just to start foreground service for some background task and then show call activity.
It is not an overlay window. Because we can re─Ćuce it like any ordinary activity.
Who tell me haw this extra ordinary call activity is chown inspite screen locked or not.

Source: Android Questions