"It really seems that in earlier years, when resources were limited, people really used their ingenuity far better than today."
We still use the same system today: most ham radio repeaters are triggered (and can be remotely configured by the owners) through audio tones; also most license free radios implement subtone codes (ctcss) to trigger the audio output when a radio with the same code is transmitting. The idea is that people wanting to talk each other use the same code so that they won't listen to transmissions by others. The system works as intended but is flawed by giving a false sense of privacy when users think others won't be able to listen to their conversations just because they use that code, which is false as there is no proteciton at all: the code is merely used on the receiver to voluntarily squelch off the audio when the same code isn't provided by the transmitter, but anyone with a receiver not using codes can listen to them.
We still use the same system today: most ham radio repeaters are triggered (and can be remotely configured by the owners) through audio tones; also most license free radios implement subtone codes (ctcss) to trigger the audio output when a radio with the same code is transmitting. The idea is that people wanting to talk each other use the same code so that they won't listen to transmissions by others. The system works as intended but is flawed by giving a false sense of privacy when users think others won't be able to listen to their conversations just because they use that code, which is false as there is no proteciton at all: the code is merely used on the receiver to voluntarily squelch off the audio when the same code isn't provided by the transmitter, but anyone with a receiver not using codes can listen to them.