Would you like to turn your android phone into a ringer phone installed at the reception desk?
This app is a simple application that only has a "call" button.
When you press the call button, the smartphone's native phone app will be launched and a call will be made to the contact set in the app in advance (= you need a SIM that can make calls).
By physically hiding the unnecessary part of the screen, you can eliminate the worry that the customer will arbitrarily hang it outside.