Build an Emergency Telephone PCB for Elevator

What I have seen that was really cool is "sound powered " phones. The microphone element is some sort ofvariable reluctance device that generates enough power to drive the receiving element with using ONLY a direct connection. I actually bought a set many years back, and they certainly are "sound powered", needing no other source of power at all. They were great for kids who did not want to frequently buy batteries.
I think fire-crews use that sort of system to communicate in tall buildings, but I have no actual verification of that.