An old man I used to work with had a set. Two old handsets with two conductors coming from them. It had a 9v power supply, I believe. We used it to trace existing interlock wires, hundreds in one conduit. Just hook one lead from each phone to a common wire, usually the ground, and try the others till I heard the ol man whistling. How can I make a set of these. Other designs require the base or four conductors. Thx
You basically want to make a line tracer. You don't need a phone at the other end, just something that injects a signal. This is what the old man did manually by whistling. You can do it yourself by hooking a signal generator to one of the wires at one end, then probing around at the other end to find that wire.
One thing to keep in mind is that the signal will capacitively couple between wires in a bundle, more so the longer the cable is. To get around this, use a low frequency, like 500 Hz, and have the receiver present a significant load, like 50 Ω. The receiver can be as simple as a powered PC speaker with 50 Ω accross the input. There will be a strong difference between the volume you get from the wire that has signal injected on it, and the others that don't.