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I’m trying to connect a modern 4-wire telephone surface-mount jack to what appears to be an original 1949 3-wire telephone line: only red, green and yellow wires. I’m replacing the original four-post jack whose which had stopped working. I’ve matched the incoming colored wires to their corresponding posts in the new jack, but I get a “no-line” indication on my phone. What am I missing?

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    Phone lines only require two conductors. You just need to figure out which two are live. Did you investigate the other end of the cable?
    – isherwood
    Commented Nov 28, 2023 at 19:11

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Tip is green and ring is red - ignore yellow. That's a very old standard.

Unless you have more than one line, only the center pair of contacts are actually used. The same modular body can have 2, 4, or 6 contacts, but if you have one landline, you're only using the pair in the center (Green/red or white(with blue)/blue per standard color schemes.)

It's possible (even likely) that the break is not at the jack you replaced.

In the very likely event that you now have a modern "Telephone interface box" outside your house with the very old wiring, it might be time to update the wire between the two. Or indeed, start with plugging your phone directly into the "customer access" jack on it to be sure the problem is not beyond the house.

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