After the dot-com crashes I took a job at a transportation company on graveyard shift and was usually the only soul in the building for a good portion of the night. Some nights I could swear I heard an old-fashioned telephone ringing, but it was so muffled and indistinct I could never pinpoint it in time to answer it.
So what's a guy to do? Wardial every number we'd been assigned until I found the right one, and then dial that over and over until I found the phone itself. I found a lot of phones in weird places; Two in closets, one under a ladies room sink, one behind an electrical panel in a storage building, etc, but finding the one I wanted was a bit harder. Even armed with the number I could only ever get close; It just seemed to echo throughout our offices.
I had to break out the big guns, a linesman's handset and a wire tracer.
Turns out that when they'd converted a portion of a disused repair shop into more offices in the 1970's the workmen managed to cover an operational telephone in drywall, leaving it to ring into a void created by the fake walls and drop ceiling.
The best part? Just feet away, hanging on the wall of the break room was an enlarged black and white photograph of the old repair shop. The phone was clearly visible and blown up to nearly half real-world size.
I only ever used my knowledge for evil, of course. If I was stuck late I'd take over a table next to the coffee maker with my reports and spend the last half hour or so annoying my coworkers by calling it and watching them scramble. If someone asked me if I knew what phone it was I'd point at the photograph.
"Maybe it's that one."