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?
Typically IR transmitter LEDs are transparent or light blueish, to allow as many photons as possible to exit the LED.
Typically IR receiver LEDs are dark, almost black, made to block out visible light and only pass through infrared wavelengths of light.
Here you see an IR transmitter LED on top and an IR receiver LED below that.