At one end of the wire is a 3.5 mm Jack plug. On the other end of the wire is presumably a glass LED.
When connected to the phone, nothing happens. The LED does not start to glow.
Also, this wire doesn't work as a microphone. It's also not an earpiece.
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Some manufacturers use standard audio connectors to carry just plain power.
They're robust and can carry relatively high current and voltage.
It works, I can see why they get used. After all RCAs are on everything for everything.
I have an e-bike that uses an XLR as a charging port for the battery.
There's an IR led on a cable with a 3.5mm jack somewhere that's an extender for my home cinema system remote.
(That might be what this is, so see if your phone camera can see the IR light from a TV remote and then test it with that thing)
This possible LED plugged into something either home made/bespoke, very old, or Chinese.
Small chance it's from some medical or scientific equipment that hasn't moved with the times.
If it's an LED put a DC voltage down that plug. If it's a light sensor, measure for a DC voltage.
Audio AC signals didn't have an effect so it's probably a DC component.
My bet, point your phone camera at it and put a DC voltage down there in the right direction and you'll see IR light come out.
It might be the receiver. In which case you need to monitor voltage. Then point a TV remote at it.