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.
What is it for?
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.