Why do I hear a FAX signal when I call a payphone?

Most payphones these days are programmed to reject incoming calls. When this is the case any attempt to call the phone will probably send you straight to an internal modem. This modem does not actually function as a FAX machine, so you will never see thermal FAX paper pumping out of a payphone handset. The internal modem is used to connect computer software (usually Elcotel) to the payphone. This software communicates with the payphone to generate reports on the phone’s health, how much money it made, etc. You can get an idea of what the software looks like starting at the 3:18 point of Ugo Massa’s Hang Up, a Payphone Documentary.