At 11:08 AM 2/6/99 +0530, Greg Cormier wrote:
>When no one is on the phone, voltage is at ~50VDC. When someone picks up
>the phone, voltage drops to ~8VDC (someone check that number?) So a 20V
>Zener would let the current through when not in use, and stop when it's
>picked up. Hook that up to an Opto-isolator or something simliar, invert
>it, and you have a phone in use indicator. As for the DTMF, maybe someone
>else can help...

If you have to do it by voltage, and you have to use a single threshold,
then 18V is what I'd reccomend.
Loads of experience, time, and number of deployed units behind that number.