An indicator that announces which electrical circuit has been active (as on a telephone switchboard) WordNet 3.0 & Open English WordNet

An annunciator panel, also known in some aircraft as the Centralized Warning Panel or Caution Advisory Panel, is a group of lights used as a central indicator of status of equipment or systems in an aircraft, industrial process, building or other installation. Wikipedia

A signalling device that shows which of several electrical circuits is active, especially such a device in a telephone switchboard. Wiktionary