Need help for my project

Hi
I'm new here and I tru to receive some help to achieve a project.
I work in a companies where everyone use phone constantly and I want to build a kind of busy light.
My goal is to install this between the phone and the headset.
So here what is my idea.
A little circuit with 2 female rj11 connections and a power 110v in and out.
A soon as we detect a tension on the line, this trun on and let 110v passe. (This will open the light)
In fact I wish to have a circuit that will be a switch that turn on if its detect tention.
I think the best way to do this is mosfet but any idea will be really helpful
Did some one can help me with this?
Thanks for you help.

Do you want to detect DC between the phone and the headset? There is no DC, instead it is AC audio at a fairly low level.
The AC audio varies in level so if you make a circuit that turns on an LED when it detects audio then the LED will sometimes be turned on and between spoken words it will be turned off.

Thanks for the reply,
My goal is to create little circuit that can be place between phone and headset and who will light a light when someone is at the phone. The reason I want to place it between the phone and the headset is because we use ip phone.
Do you have an idea to do this?
Thanks by advance.