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?
It is an infrared receiver extension cord.
Say you have a device that is controlled by an infrared remote, but is tucked away out of line of sight.
You can use this cord to move the infrared receiver into sight.
Here it is on amazon : https://www.amazon.com/uxcell-Infrared-Receiver-Receiving-Distance/dp/B07FKMK3GZ
Can’t it be used as a replacement for the infrared port (if the phone doesn’t have one)?
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.
It’s a receiver, not a blaster
No, since your phone only outputs audio through the headphone jack. These required a PC card that used a jack that’s wired up to use this.
Audio jack ir blaster adapters for phones used to be common. You can still buy them on eBay.
Audio is voltage changes which can be set to drive an ir led. You use an app that outputs audio that matches the ir signal protocol.
Removed by mod
Yes but nowadays you’d buy a USB version since few phones have headphone jacks. They’re on Amazon.
Here’s an old style headphone jack ir blaster adapter:
https://www.ebay.com/itm/404693924792