I mean the ruskies cooked up this guy:

https://en.wikipedia.org/wiki/The_Thing_(listening_device)

Invented by THE theremin. Yes, the instrument, theremin, the wacky one where handwaves and suck around some metal stick make funky horror movie noises, there was a Dr. theremin who invented that, and also this. Anyway, it was a "passive radio device" (given as a "gift") whereby if radio waves are projected onto the device it shall convolute said waves with sound waves of the room and then back-project a signal of radio waves with voice data overlayed, which can be read. Fancy at that sounds it is trivially low tech and simple. Very analog.