I wish to extend my incoming telephone connection using CAT5 as I've heard it could provide better performance than standard single stranded telephone wire over long distances.

There are only two wires in the phone wire, do I just connect these to any two wires in the CAT 5 cable, or do I need to treat the twisted pairs in the CAT5 as a single wire - ie connect both blue+striped blue to one wire, and green+striped green to the other wire.

In short, to get the best out of the CAT5 cable do I need to take the twisted pairs into account, if so how?

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    \$\begingroup\$ The telephone company runs the connection to your house over many miles of cable with substantially lower grade than CAT5. As long as your existing indoor wiring isn't any worse, switching those last few feet inside your house to CAT5 isn't going to make any difference. \$\endgroup\$
    – brhans
    Commented Oct 8, 2018 at 14:46
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    \$\begingroup\$ @brhans Makes me think of gold plated fuses and very expensive mains cables for audiophiles. \$\endgroup\$
    – winny
    Commented Oct 8, 2018 at 16:28
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    \$\begingroup\$ the incoming phone line is a twisted pair .... just keep the twisted pair going all the way to your phone ..... do not connect separate untwisted wires \$\endgroup\$
    – jsotola
    Commented Oct 8, 2018 at 17:04
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    \$\begingroup\$ This actually does depend on your region. e.g. in Australia we use a different set of colours for the "second pair" compared to the US. It's best to follow your country's standard practices to reduce future confusion (and sometimes it's even a legal requirement!). \$\endgroup\$
    – Bob
    Commented Oct 9, 2018 at 0:14

4 Answers 4


A CAT5 cable has 4 twisted pairs. You will want to use 1 pair of wires per phone line. The standard arrangement is the blue pair is the first phone line.

Assuming the wiring your connecting to is the standard Green/Red/Yellow/Black used in the US, and that it's connected correctly, connect the existing Red wire to the Blue with White wire, and Green to the White with Blue.

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In the US (and pretty much everywhere else, though I won't claim to know how things are done in very rural or poor countries), the telephone cable plant (the run from your home to the central office) is twisted pair, not "single stranded". It's just not as tightly controlled in spec as Cat 5.

You need to connect the two wires of the incoming pair to the two wires of a pair in your cable.

The incoming cable probably has two pairs to allow you to have two phone lines in your home. In this case, you should probably connect both pairs to your incoming wiring to avoid confusing a future installer when somebody wants to add a second line (not that that is super likely in the cell phone age).

As other answers have mentioned, 10 or 100 Mbps Ethernet leaves two free pairs in a cat 5 cable, specifically to allow for connecting voice circuits.

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    \$\begingroup\$ In our 1990s US house, the builders installed non-twisted wiring, basically the same stuff you use to plug into the wall. When I put in CAT 6, I saw a 20% improvement on our DSL rates. In-building wiring matters because it is unshielded and runs close to electrical lines, motors and fluorescent ballasts. \$\endgroup\$
    – user71659
    Commented Oct 9, 2018 at 0:16

CAT5 cable has four separate twisted pairs. Use the two wires in one of those pairs for your telephone connection.

10/100 base T Ethernet uses the green and orange pairs, specifically so that you can still use the blue and brown pairs for telephones.

  • \$\begingroup\$ Just to be clear, are you saying use only two wires (from the same pair), to connect to the two telephone wires, or are you saying use two pairs (eg the blue pair and the brown pair). \$\endgroup\$
    – userSteve
    Commented Oct 8, 2018 at 15:28
  • \$\begingroup\$ I really don't know how to say "the two wires in one of those pairs" any more clearly. Do NOT use two pairs. (You could have one phone connected to the blue pair, and a separate telephone connected to the brown pair, however.) \$\endgroup\$
    – Dave Tweed
    Commented Oct 8, 2018 at 15:46
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    \$\begingroup\$ Just to throw out a wording: pick a twisted pair. Use both wires in that pair. Don't use any other wires =) \$\endgroup\$
    – Cort Ammon
    Commented Oct 8, 2018 at 22:52

The twist in twisted pair helps reduce EMI and crosstalk for differential signals on that pair. If you use both wires in a pair as a single parallel line you lose the benefit of the twist and defeat your own purpose. (e.g Blue&Blue/White tied to red, Brown&Brown/White tied to green).

Use Blue and Blue/White as line one. Brown and Brown/White as line two.

You can still run 10/100 ethernet or two other lines over the orange and green pairs.


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