ISDN is a set of communication standards for the simultaneous digital transmission of voice, video, data, and other network services over the digitalised elements of the Public Switched Telephone Network (PSTN). In telecare, ISDN services have often provided telephone connections to alarm-receiving centres. Confusingly, although the definition of ISDN includes the word “digital” the service does not use the Internet Protocol. ISDN services are being discontinued as part of the upgrade to IP-enabled digital phone lines and so must be replaced.