Red said:
Can one use the unused wires in a network CAT 5 (and 5e, 6) cable for a
telephone connection?
Yes, that was the intention.
Have you ever noticed that an RJ11 plug actually fits into an RJ45
connector?
Well, it does.
And telephony uses the centre 2 pins on the RJ11.
Plug the RJ11 into the RJ45, and the telephony is now
on the centre 2 pins of the RJ45 ( 4+5 )
Ever wondered why ethernet uses pins 1+2 one way, and 3+6 the other?
Seems odd pair splitting.
Well, there's the answer.
Because pins 4+5 were reserved as a pair for telephony.
I've never actually seen this in use ( telephony on 4+5, ethernet on 1+2,
3+6 ) but that was the original intention, so a single wiring system could
serve the phones and data. It may not perform too well with 100Mbit
ethernet, due to crosstalk. I don't know, I've never tried. I also think
gigabit ethernet perhaps requires all 4 pairs for data.