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One of my customers has build a new house with pre-installed cables for internet. The only thing he didn't think about was installing a cable for his telephone line.

Behind his desk, he has a "spare" UTP cable, my question.

Can this cable be used to replace a telephone line.

This client has a internet and telephony contract, using a experiabox phone/internet modem.

Any help would be great. I knowe I have to use a RJ11 plug.

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up vote 7 down vote accepted

Spare Cat-5/5e/6? Sure. If you need RJ11 instead of RJ45, you'd need to snip off any existing RJ45 connection and replace it with an RJ11. For a single phone line, you only need 1 pair (2 wires) out of the existing 4 (but use a matched pair). For a 4-conductor/2-line RJ11, use 2 pair. For a full 6-conductor/3-line RJ11, use 3 pair.

Here's a wiring guide (source):

enter image description here

The colors are somewhat arbitrary; you just need to make sure you the the pairings in the right order at both ends.

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+1. Good answer. When installing new network\phone wiring I always use CAT5e UTP as it can be used for both phone and network. It makes it so much simpler knowing you can use any jack for either one. – joeqwerty Jan 9 '10 at 19:47
Great answer, I think your answer saved me a lot of phone calls. I asked this because I thought there could be to much line resistance and signal distorsion. – Chris Jan 9 '10 at 20:58
Additonally, if you're in a pinch, an RJ11 plug will physically fit, click and work in an RJ45 jack. Many moons ago we switched an ethernet run over to a POTS phone line for testing modems, and have been using it with the RJ45 jack for years now. – Ƭᴇcʜιᴇ007 Jan 9 '10 at 21:07
I knew that, but thanks anyways. – Chris Jan 9 '10 at 21:38

Just buy a cable that have 1 rj45 and the other end 1 rj 11 But if you buy ip phone just connect it, be sure to change the swich from data to voice

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