My schools had two way inter coms. I think each classroom speaker had a cable set going to the office, and an annuciator device to show which classroom called. The older section of the school had speakers above the blackboard, anfd the switch below or beside it. Newer sections of the school (built after the mid 1960s) hade a speaker box with a switch on it. The vocational shops had a telephone like hand set and a cradle with a switch, and a horn PA speaker.
* if things are right, your toaster could emit music causd by thaudio signal going through the heating element.
** FM on cable. Done in North America too, at least until the mid 1990s. You could often hook cable up to your regualr stero system receiver. Many had a 75 ohm or 300 ohm input.
