In another forum I'm on, a member once mentioned:
"Back in the days of the Bell System, the Telephone Company leased equipment and charged you for how many phone extensions you had connected to your line. This was done by measuring the impedance imposed by the ringer on each phone connected. Some people who "obtained" additional phones without the Phone Company's consent would disconnect the ringers to make those extensions "invisible" when the Phone Company tested their line.
When I “acquired” an extra Western Electric 500 desk phone, I wired a fluorescent starter in series with the ringer in that phone. The “signal” used to make a landline phone ring is typically a 90 volt alternating current at a frequency of 20 hz. The lower voltages used when the Phone Company tested the line was not enough to ionize the neon/argon gas in the starter so the ringer in that phone could not be detected. But when an incoming call was received, the higher ringing voltage would ionize the gas in the starter, which would then close the circuit to the bell and allow the extension to ring."
Genius!, I've encountered a few rotary phones (both in storage and in service) that have a label claiming that they were rented or leased out.
