Now here’s some logic that should work fine with the right zener and the right resistors and a couple of cheap npn’s 2n2222A’s or 2n3904’s (06’s?). If you get close to 25 volts with the new smart test boxes, a 20 volt Z may work fine. Choose R1 to limit current through Z and have enough left to turn on npn1 just enough to deprive npn2, choose R2 for that, and you will need to add a resistor R3 to protect the LED from overcurrent as needed, depending on the phone system you have!
You MIGHT need a resistor between the bottom of R2 and the base of npn2 to get it right, but I don’t think so because of the B-E 0.7 volt diode junction voltage needed in npn2 to get it out of cutoff.
- Richard Steven Walz mwandel[at]bnr.ca