UPDATE: 20191230 - Uploaded a copy to my Peertube account.
From time to time I carp about how generally lousy our bandwidth is out here. Verizon (our CLEC in the Bay Area) has all but given up on maintaining their infrastructure out here, aside from the bare minimum to keep the copper from turning to verdigris. They gave up on deploying fiber some years ago (local mirror) some years ago, and from the poking around I've done on their side of the fence, their general stance in the Bay Area appears to be "Get everyone on celllar so we can ignore the rest of the network." Which sucks and does nobody but Verizon's shareholders any good in the long run.
Anyway, after yet another afternoon wasted on the phone with tech support because our speed fell to pre-dialup speeds for reasons unknown, I decided to take the bull by the horns and put some old skills to work. Out came the fox and hound and my old lineman's test set, and I set about figuring out which lines in the fist-sized morass of ancient wiring outside, if any, were actually hooked up. The way a fox and hound works is, you clip or plug a tone generator (the fox) into the line you want to trace, and you use a matching inductive probe (the hound) to listen for the sound. Telephony cables are almost never insulated so you don't need to touch the copper directly, the faint EM field around the wire is sufficient.
I was able to trace the line successfully, but in so doing I found out why our bandwidth was so terrible. Thankfully, after demonstrating the problem to the contractor that Verizon sent out, we were able to work together to not only rip out the dead cabling outside, but mostly resolve the interference.
Postscript: It still strikes me as strange that you can just order telco gear off of Amazon these days. Way back when, the only way to get any of this stuff was the old-fashioned way.