1) An announcer 2) Indicator 
• (n.) An indicator (as in a hotel) which designates the room where attendance is wanted. • (n.) One who announces. Specifically: An officer in the church of Constantinople, whose business it was to inform the people of the festivals to be celebrated.
an indicator showing remotely whether each of several items is in the required position or state or not, e.g. door signal with automatic doors or lamp indicating any of several abnormal conditions
An·nun'ci·a`tor noun [ Latin annuntiator .] 1. One who announces. Specifically: An officer in the church of Constantinople, whose business it was to inform the people of the festivals to be celebrated. 2. An indicator (as in a hotel) which designates the room where attendance is...
[n] - an indicator that announces which electrical circuit has been active (as on a telephone switchboard)
noun an indicator that announces which electrical circuit has been active (as on a telephone switchboard)
an announcer. · a signaling apparatus, generally used in conjunction with a buzzer, that displays a visual indication when energized by electric current.