🚀 go-pugleaf

A friend recently showed me a "technique" for making free phone
calls from a payphone.

I don't want to promote this method---I just want to know what 
how the phone hardware/software allows this to occur.

Let me try to describe it---it's difficult without visual aids,
but hopefully you will know what I am talking about.

step (1): get paper clip, and straighten it out.
step (2): puncture the metal cover that is underneath the 
          part of the phone you speak into---specifically, poke
          it through the hole closest to where the cord connects
          to the receiver.
step (3): insert paper clip into the puncture, and scratch the exposed
          end against a metal surface while dialing your number.

[Note: I have not been able to do this succesfully myself---but a random
sampling of three payphones in the U of Chicago Science Library found
that all three had the tell-tale hole of step (2).]

So---How does it work? (And does it work?)

Thanks,
Barry Merriman
Grad Student, U of Chicago Math
barry@zaphod.uchicago.edu