A second chapter to this story (which does not really involve me) was that this same person told me of a 'radar test set' which *COULD* be used to detect unauthorized wiring and even unauthorized sets which had ringers disconnected.
This was apparently a TDR device (I was familiar with the technique of testing transmission lines using TDR) and their unit only had one of them, which they usually used for such things as finding opens, bad splices, forgotten bridge taps, etc. on cables.
He said that they would occasionally use it to gather some evidence on suspected hardcore Phone Phr^H^H^Hhobbyists, but it was seldom done.
Good example of 'social engineering'! ;-)
My impression was that Ma Bell went for the low-hanging fruit in cases of unauthorized sets, but was well aware that many subscribers had them and that there really wasn't much they could do about it.
'Phone hunting' was a frequent pastime among the Phone Phr^H^H^Henthusiasts in those days, such things as seeking out stray sets in recently-vacated apartments, hotel elevator lobbies, etc. There was really no shortage of feral sets out there in the wild !! ;-)