... and the occasional fax machine, just barely audible above the noise floor. There were some weird things the local switches would do that I never figured out (seemed like USWest hadn't set them up correctly, or some weird fault conditions (I was doing weird stuff to them.))

Being an electromechanical switch, there is a direct electrical circuit between your handset and anyone else you call on the local exchange. So, if you attach a battery to your phone, the person on the other end of your call can't hang up. (You can measure the voltage on their end with a multimeter)