Interestingly enough I had already read your queries and answers. Had your diagram up on my screen.
However it is true that our phone system is a business phone system. As one person I think appropriately said, business phone systems aren't really phone systems. That is, they are proprietary hardware and sometimes software build by any number of companies. It seems in my research that this is a particularly hard nut to crack generically, in other words one solution will not fit all. I do have an answer however that Im trying to pursue. I alluded to it earlier in my response to one poster. I think an induction type device would work. The idea came to me when I saw various circuits for Inductive phone taps. (
) Of course I have no interest in taping anyones phone, and the way I would build this thing it would be pretty obvious on anyones phone. But I thought if I could simply put on the hand set wire an induction device to pick up voltage when the phone is on line. Possibly increase the voltage using transistors to a flashing led it would simply pick up the voltage, light a flashing led and voila there you have it. Its passive, and actually should work on any type of phone system. Alternatively you could do this and use a 9 volt battery to flash the light.
I'm a novice so anyone who would like to diagram such a things feel free to post some ascii art circuits.