This is CADRSTATION. Special purpose old landline phone exchange for small households and hackerspaces. It requires only one rotating stepping switch. There is a difference compared to other exchanges: here all telephones are connected together all the time and talk to each other in a big multi-conference style. The rotating switch is used to send ring signals. Optional external connection to the city phone socket is planned.

It bears CADR hackerspace logo, because it was assembled and tested there.

And here is schematics (ask me, I will tell you more):