500 Series Desk phone with mechanical rotary dial. 2500 Series Desk phone with Touch-Tone keypad. The ringer on these 2 series phones have 4 wires: Ringer Wire Terminal Red L2 Black L1 Grey (or Slate) K Red/Grey A Before the breakup of the Bell System, some Phone companies would check each line and determine/calculate the number of ringers attached and would charge for additional phones. Many "bootleg" phones were installed over the years, with the bell disconnected. When a phone is hung-up, or "on-hook", the bell is the only thing still connected to the telephone network. With the use of "modular jacks" and "Network interface" boxes between your home and the telco network, I believe checking for ringers is a thing of the past. Sometimes it is nice to have that extra phone ring again ...
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