ISDN telephone systems, or “ISDN Telefonanlagen” in German, are a type of telecommunications technology that uses digital signals to transmit voice, data, and video over traditional copper telephone lines.
ISDN systems offer a number of advantages over traditional analog telephone systems, including faster data transmission speeds, better voice quality, and the ability to support multiple devices on a single line. ISDN was particularly important in the early days of the internet, as it provided a way to connect to the internet at speeds that were much faster than what was available on dial-up connections.