I forgot to ask in my earlier post . . . Did Heathkit ever sell telephone equipment?
Back in the late 1960s some electronic supply stores began to sell telephone sets, perhaps older 302s or non Bell products, such as AE or ITT. Around that time people discovered they could buy their own telephone set, with the 4-prong plug, and not have to pay the extension rental charge. (A set cost about $10 back then). The bootleg extension market got rather large.
Bell, as well as the state regulators, naturally didn't like this and threated the wrath of the heavens upon anyone discovered with a bootleg phone. _Supposedly_ Bell developed line testers that added up ringer resistance on a phone line and compared it to its records to see if there were bootleg extensions in use. Of course, word of this got around and consumers disconnected the ringer of their bootleg phones.
While Bell literature described such devices, I question if they really existed or were used to any great extent. The state of the art of computerization and equipment back then meant comparing business office records to a line tester wasn't so easy since the interfaces were usually very different. Converting AMA punched paper tapes to a form the billing computer could use required translation steps from one medium to another. I also question whether subscribers' service was disconnected--beyond a few "to be made an example of"--for having a bootleg phone.
Also some electronics stores sold parts, like headsets, dials, etc. A friend of mine bought a headset (the 52 series?) and spliced it into his phone. I did likewise. We both found we could hear fine, but transmission to other people was poor. I wondered if headsets required a different circuit than the 500 set. At the same time my friend experimented with his phone dial, changing it to 20 pulses per second and found that it worked, nicely speeding up his dialing time. (we were served by either panel or #1 xbar).