Yes, you _can_ use a single 4-pair Cat5, but every time I've seen
this done, people have lived to regret it. Commercially, I've seen
installers do this and then get burnt down the road: going from a
Panasonic analog KSU system to a digital KSU often requires going
from one pair to three or four, for example, or things like the
early 100baseVG (I think... 100mbps on 4-pair Cat3, whatever it
was). The long term cost far exceeds the short term savings, as
it costs like hell to have the cable guys come back a second time.
Even residentially, I don't think it makes sense, even though your
time is cheap, and the wire is (relatively) expensive. Consider:
1) You will find yourself creating a Charlie Foxtrot at the central
termination point. Network is typically terminated on RJ45
patch panels, and telephone on 66-block. You have to run two
pairs of each cable to each, which is ugly and potentially
failure-prone
2) You potentially destroy some of the qualities that make your
cable Cat5-compliant, which may create additional noise and/or
errors on your network
3) Your phones (particularly KSU-less multiline phones, in my past
observations) may pick up some crosstalk from the network pairs
as noise, which can be irritating
4) You lose the ability to use the "unused" pairs in the future to
rapidly meet a short-term or emergency need - you MUST run new
cable to get additional capacity
5) You may not be able to use the circuits for other things, such as
T1, serial-on-RJ45, or other RJ45-format services
I don't think it makes sense. Do the work, do it once, reap the
benefits and be done with it. If you decide to do it, be sure that
you've carefully considered the five points above, and that none of
them will be a concern to you.
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