This is from the security training guides for Norway public services in the 1970s(?) It may not seem “PC” but the glamourous young secretary is smarter than her boss as she remembers the warning “some telephones can be made to work as a microphone even if the handset is on hook. So unplug the cable!”
A German professor explained how it worked and that the East German phones all had the extra bug wire added in the phone socket (although all European phones could be similarly modified) and suggested “Try it!”
so I did :laugh_at: (I used a more modern version of the 700 series with an electronic mic as the older ones are too noisy)
and then set up the “monitoring post” below. The red/green lamps/relays are what would be at the Telephone Exchange; they only light if the line is seized. The blue telephone is the same kind Openreach use to test lines with, it can monitor them without making a noise on the line or when the audio is very quiet…
It is possible to still ring the line so no one would notice anything is unusual; except I don’t answer it (as it could be spies! :laugh_at:) and have the “ja ja ja polka” playing by the phone “volle Kanne” (as I needed to make some noise where the phone was) but it would stop the audio monitoring of any other noise in the room 😉
