With a single POTS telephone line and two or more handsets, when a call comes in, each handset can join in with the call. This can sometimes be an issue when one user doesn't know the line is in use, and picks up the phone and starts dialing, or listens in to the original call.
Doing some research, I've found that there is a nominal 50v on the line in idle, the ringing is a 25hz wave superimposed on the 50v, and normal "off-hook" voltage is ~10v. Telephone Technical Information
What I'd like to do is detect the 10v and light an LED to give an indication of this "off-hook" call-in-progress state.
I suspect a simple circuit powered by a AA battery should be easy enough to do, but not sure how to go about it, nor have I found anything that does something similar that I can hack together.