Should long wires be kept separated from each other, to reduce interference?
A benefit of using CAT-5 is that the pairs are twisted.  You would want to look at how you are using the pairs so that you're not introducing cross talk inadvertently, but the twisted pairs tend to resist RFI which is why its used.  Telephone wire, however, can be a completely different story.  You may find some that is twisted pair, but most if it is simply flat cable.  Worse, much of it is very cheap flat cable with what amounts to a piece of thread with a very thin foil wrapped around it.  This is very high loss and very vulnerable to RFI.