Siemens and Halske French intercom phone

The five buttons on the left are labeled for the different levels of a building: Parterre (Ground Floor) and I. Etage through IV. Etage (1st through 4th floors). The circular metal component in the center is an open-faced ringer (or buzzer) featuring two coils and a striker, which would sound when a call was received. The terminals at the top are marked Z, K, S, P and 1, 2, 3, 4. In German telephone engineering of that era, these stood for Zusatz (Additional device), Klingel (Bell), Sprechen (Talk), and Phon (Phone/Receiver), with the numbers corresponding to the specific floor lines.