A step-by-step automatic telephone exchange brought into operation in 1921 at a post office in Ljubljana, which was the first automatic exchange in the then Kingdom of Serbs, Croats and Slovenes (later Yugoslavia). Originally it connected 500 users, but the facilities were later upgraded. Before being donated to the Museum of Post & Telecommunications in 1981, it had a total of 2000 subscribers.
Two automatic telephones are connected to the exchange; the first is a Siemens & Halske and the other Telefongyar R. T. Budapest.
