The label reads "Brünn", before the date. So that tells you that this phone is from the area in the Czech Republic that was occupied by the Germans early on in WWII, actually before WWII proper. Brünn was the German name of the city of Brno. And this is not that far away from Vienna, just a little bit north of there, probably not more than an hour drive.
So, I would research who was making telephones in the area. I suppose the phone is likely German or Austrian from the time 1938-1945.
If the green button is indeed original than it could very well been made in that period as plastics came in all kinds of colors by then. Even Polyethylene was established by then.
The design of the housing seems actually quite finished, the inside is smooth and clean with metal inserts for screws. Seems like a finished production unit.
The dial mechanism is interestingly different. How many pulses does it produce? 12 like the Siemens and other designs of the time? -- These dials have another contact that cuts out 2 pulses.
The style of the gongs is interesting too, not the usual round type, but this oval type, which I have also in a Siemens telephone with a message unit counter. The shape save space. This shape might be just a little different. Have to compare.
That is indeed someone's name on the label, the person who inspected the telephone. Kontr. clearly means "Kontrolle", or "Kontrolleur", in this context translates as "inspection" or "inspector".
I think this clearly establishes place and date of manufacture.
PS: I think the name Brünn could have been used even outside the period of German occupation, if the company making the phone was German in origin, as the name is goes way back in history and was probably used by many ethnic Germans in the area. So the period I mentioned is not exclusive. But it's almost mood, because you have an actual date.