Old circuit i've seen involves one rectifier (B250C1500) with the output
across a 22k resistor and a high efficiency diode.  With a line voltage of
~40V coming from the phone lines when not busy and the LED will light.  The
rectifier means that it doesn't matter which way round you have it, also
polarity protection is of no consequence.  When in use the LED will dim.
Using a zener as mentioned in another e-mail will stop it lighting when the
phone is in use.  This could easily be altered for use with a op-to isolator
for input into a PIC for timing.

PS.. In most countries it is not allowed to connect anything other than
approved equipment across the phone lines.  Hence talk to the phone company to
see if you can get the Okay.  They check using line resistance or something
like that.

I hope this is of any use.

Karl  U.