What I would like to do is use the existing cable and have a phone
> line run out behind the tv. I do not believe all 8 wires are used in
> 100 MBit Ethernet.
Only two pairs are used for ethernet. If your cables are wired in a typical way (T568A or B) then the first pair (blue, in the center) and the fourth pair (brown, on one side) are not used and you can hook up two phone lines or a second ethernet run.*
N.B. this is not a recommended use and should not be done if you are installing the cable professionally but I have never had any trouble stealing the two extra pairs for my own in-house purposes.
I have seen a "splitter" that turns one ethernet cable into two connections by using the extra pairs** but I have never seen one that breaks out telephone on the spares; you'll have to do that one yourself.
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