If I am on a call, the top of my phone screen displays a thin green bar across the entire screen.. It has nothing to do with time although the time display is in the center of the bar. When I am on a call and go into an app, the thin bar is shown on the top of the screen. I press anywhere on the bar to get back to by phone display. Have you tried clicking on the Phone icon to get the phone screen display back? It also occurs to me that perhaps the app isn’t being completely quit. Try swiping up on the app to close it out and then click on the green top bar.