Not to neglect above-ground ...
Remember when Hillary Clinton was found to have used numbers at the White House that the phone company said didn't exist?
Believe me, there are NO numbers the phone company doesn't know about, but there are such numbers.
We once got stuck with one of them. The phone company said that we also owned a State Department number. For a few days our little shop in Virginia was collecting calls for a classified entity within State over at DC. Laughs all around on that one. Then they disappeared into the bowels of telephone land and we never had that problem again.
There are a number of other strange telephone stories I could tell you about ~ like the need for an armed detachment of soldiers to guard the guys cutting into the welded manhole cover to get in there and replace a device that'd screwed up one of my lines.
That's really common in the DC area. Makes it hard to get service on lines like that ~ tell me about underground wires!
Not to neglect above-ground ...