Initiate a call with Zoiper beta
During the call close its window with Android "window/task manager" (with a swipe)
Now try to put the call on speaker or just try to hang it up.
Since the call is not displayed on the status bar (as it's the case with regular phone calls) it's not possible to get back to the phone controls. Hanging up the call is only possible by force-closing the application from Settings.
Android version: 8.0.0