On Sat, 11 Mar 2006 23:13:11 -0000, Dave Platt wrote:

http://www.hobbytron.com/CK007.html is one such.

A better circuit - one that I've used as far back as 1980 for
exactly that purpose - consists of a R/S 5 v reed relay in series
with one leg of the phone line. "Standard" telephone line off-hook
current is 30 mA, which will trip this relay and not degrade the
"battery" voltage at the phone. Hook up a 12V "wall wart" and LED
in series with the NO contacts and you're in business.

Mind you, this doesn't cut off the extension - it merely indicates
that the line is in use. To cut off the extension there are other,
more sophisticated (and expensive) devices.

--
73 de K2ASP - Phil Kane