my wireless headset for my office phone has an accessory called a "BusyLight" that glows when you are on the phone. they want $47 bucks for it!

so i grabbed my trusty oscilloscope and found out it outputs 3.9v DC while the headset is active

designed this to hold one high-bright LED, and a resistor, connected to a cable that runs all the way back to my headset's base station.

so the idea is:
While on a call, the phone outputs 3.9v DC
runs through a cable
to an LED and Resistor, sitting on top of my desk / cubicle wall
so that walk-up people can see im on the phone, before they interrupt me

made using quick geometric shapes in Tinkercad.