Wikipedia
Redcare is a widely deployed standard from BT, used in the UK to allow alarm systems to be continuously monitored from an alarm provider's "Alarm Receiving Centre". Installed correctly, the system can be part of a system compliant with EN50131 Grade 4, the highest security grade in the European Standard for Intruder Alarms. The basic system involves transmitting a continuous signal on a standard phone line, which can also be used for normal voice and broadband services, to prevent alarm systems being defeated by the line being cut. Enhanced versions exist that also use GSM (mobile) as a backup to the main phone line or that can function using either ISDN or IP as the primary connection.
Although early Broadband deployments in the UK were incompatible with Redcare, this problem has now been resolved. Redcare works below the voice frequency spectrum on a phone line and therefore does not interfere with phonecalls or ADSL (which uses frequencies above the voice spectrum).
Despite many years of preparation with the first trial installations in February of 1988, in October 1991, an individual known as 'MrHappy', a notorious UK based phone hacker, managed to bring down the entire British Telecom Redcare network just weeks after launch by intentionally connecting a 24V 'Bath' type power supply to the telephone lines of a Birdseye factory in Grimsby, gaining access via the TSI method. This set off an alarm at the local exchange on the WB1400 system which at that time was still in service due to the relatively recent ending of the Cold War (see Four-minute warning for details of the network). In turn, it triggered a sequence of events, which, for reasons intended to reduce non essential network traffic during a nuclear strike, sent messages to all Redcare nodes to disconnect from the core network. 'MrHappy' was never found, and shortly after the UK early warning system was almost completely dismantled, removing all possibility of copycat attempts.