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After moving into our house (built in mid-80s), we noticed a blank wall plate right about where you would expect a wall phone jack to be in the kitchen. I opened it up and found a loop of 3 pair telephone wire. I cut into it and hooked up the green and red wires from both sides to keep the loop intact, but had no dial tone. Opening up our other phone jacks, I discovered that the jacks seem to be wired in series, with the incoming pair and outgoing pair altering between red/green and blue/white (see image below). After using the blue/white pair instead of the red/green pair, the jack worked. Is there any reason why this would have been done originally? In particular, is there a reason for me to go back and restore the pattern by switching out the appropriate wires? Phone jack wiring

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As you have found, traditional US household telephone wire contains two pairs of conductors. This was normally used to allow two telephone lines to be brought into the house, each using one pair, with both lines available at all places to ease switching back and forth or to let phones with additional switches (or, later, circuitry) switch between those two lines.

Red/green was usually the primary line, buy it was common for some wall jacks to swap the pairs so a single-blind phone would connect to the other line.

In older buildings, where the phone wiring has been changed repeatedly, the two lines may have gotten cross-connected in ... creative ... ways. Ma Bell was somewhat notorious for sloppy, least-effort solutions.

(Alternatively. rare now, you may also occasionally see the second pair just connected to a transformer. These were used to power the lights in "Princess" phones.)

  • The wiring seems to be all of the same vintage and the house is only wired for one line, as all jacks have nothing on the yellow and black pair. Is there any reason they would have switched between which pair they were using for line one other than just making life a little more difficult to someone wanting to add a jack? I assume they used the blue and white pair instead of the yellow and black pair in case someone ever added a second line later. – jgd Aug 9 '16 at 18:55
  • No clue. It made sense to someone at the time. – keshlam Aug 9 '16 at 19:24
  • Red green yellow black are colors in untwisted quad cable. Twisted pair uses pairs--blue/blue-white, green/green-white, brown/brown-white, orange/orange-white. Which type of cable is it ? @jgd – Tyson Aug 9 '16 at 21:29
  • Adding more: in quad cable typicslly red/green is used for the first telephone line, when twisted cable is used the first telephone line is typicslly on the blue/blue-white pair. – Tyson Aug 9 '16 at 21:53
  • It's untwisted 6 wire. Red, green, yellow, black, blue, white. It looks like they ran the same wire all the way through, cut it, then wired the jacks in series, alternating which pair they were using from jack to jack. – jgd Aug 10 '16 at 11:49

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