[mod.telecom] Looking for a line in use circuit

BEC.SHAPIN@USC-ECL.ARPA (Ted Shapin) (12/16/86)

I would like to build a LED driver that will light when a line is in
use.  I guess I could use a CMOS inverter and drive it from the line.
Does anyone have a circuit?
Ted.

[DAK sells this circuit as "Drew's Brainstorm" for ~$10.  A 9-volt
 battery eliminator is recommended by yours truly.  -Elmo]