What is this device on the facade?

Dear forum members, we are renovating the facade of our old building. Now the question arises, what kind of strange device has been installed next to the exterior sensor until now. Is it related to the old alarm system or the oil heating? I would be very grateful for competent help. Have a nice evening! Jakob

I would also consider an ApL (Abschlusspunkt Linientechnik) possible here, there are a thousand forms of them. The context would then be of a telecommunication nature.

Yes, it is a very old connection from Telekom

Yes, it is a very old Telekom connection

Old in the sense of "not the newest model" - but probably still active in operation (and not allowed to be opened by laypeople).