Back in the day they sold transformery things you could glue to your phone base to record calls. I'd love to know if those worked, and if any were ever sold to non-weirdos.
Your phone handset has a loudspeaker in it. Paper cone attached to a coil of wire, wiggling in the field of a magnet. Put another coil inline with it, and the varying magnetic field creates a voltage, just like a microphone would. Commercially available as a little device with a suction cup for ease of attaching to random telephones. Popular with journalists and the like.
At some point in the days when everything was orange and brown, some clever person realised that if you equipped a hearing aid with such a coil of wire, and a switch labelled 'T' to select it rather than the usual microphone, the user could hold the telephone handset up against their hearing aid and hear the call without any risk of squealing from acoustic feedback.
This quickly became a standard feature, and another - slightly less clever - person came along and reasoned that if you filled a room with a varying magnetic field, you could send audio from a microphone or television or whatever directly to people's hearing aids, improving the signal-to-noise ratio by eliminating background noise. The 'Induction loop' was born - literally a piece of wire around the edge of a room, service counter or similar, fed by current modulated by baseband audio.
Unfortunately, it stopped being the 70s, and the real world started to fill with devices that produce stray audio-frequency magnetic fields: CRT screens, DC power supplies, dimmable lighting, etc. etc. Hearing aid users learned to ignore the 'T' setting, and people forgot all about that loop thing. The microphone got unplugged by the cleaners, but because everyone likes to help the disabled, the sign indicating that a loop was available remained in pride of place.
Now the telecoil is an optional mode that you have to ask the audiologist to enable when programming the hearing aids. And most people don't. Hearing aid manufacturers are, however, in a race to see who can come up with the most irritating implementation of The Devil's Other Radio
[1].
Meanwhile telephone calls can easily be recorded by writing a copy of the data stream to a file.
And LEDs come in blue now.
[1] Bluetooth.