With iOS 11, iOS 12, calling the police or the ambulance in case of need is simple and above all very discreet. It is enough to know the trick, but it is not very obvious.
With the latest versions of iOS, just press the power button for 5 consecutive times, because three sliders appear to turn off the device, to open the Clinic Folder or to make an emergency call. In the latter case, depending on where you are in the world, a call will start at the default number 911 in the USA. Furthermore, with iOS 12, a very noisy sound warning starts.