A circuit-switched telephone network system that allows both voice and data services to be simultaneously delivered over digital lines. It was launched by BT in 1986 to migrate outdated landline technology to digital. It is now heading to end of service and limited availability.

This is the term given to a group of analogue telephone lines, primarily used by an ARC, to receive analogue alarm calls from social alarms - this analogue lines will ultimately be replaced by SIP trunks