The ongoing snail's-pace digitsation of Cassells Household Guide provides occasional gems of information. Perhaps you've seen 'speaking-tubes' in Victorian-set film or TV drama ... basically just a pipe with cones on the end, through which one could talk to the servants in the basement, without the nuisance of going onto the landing and shouting - a primitive intercom, if you will. However, how did you differentiate between sound coming down different tubes? Here's how:-
Speaking-tubes, again, are very useful, and easily contrived ordinary iron gas-pipe answering the purpose nearly as well as gutta-percha, and at a much less price, and the flexible ends and whistles can be purchased sufficiently handsome for the most elegant apartment, and sufficiently cheap to be within the reach of the most ordinary purse.
When several pipes terminate in the same place, the whistles are fitted with indicators - little ivory rods which are blown out when the whistle is used, thereby showing where the attention is required.