Integrated Services Digital Network, also known as ISDN, is a type of telephone system that carries both voice and data in digital format rather than waves.
In recent years, new systems have been able to improve upon the capabilities of ISDN. Because of this, ISDN connections are being switched off across Australia. Telstra, in cooperation with the NBN, has ceased the sale of new ISDN connections and services. Forced disconnection will start in September 2019 and will be complete by 2022.