For what purpose? Regular old phone - no advantage. IP phone network with a local exchange maybe. L
Remodels/new construction these days are starting to just run CAT5 to everything since it's not too much more expensive and adds flexibility. However, the punch down blocks I've seen as a result of this are always a mess with things cross-patched all over the place and for some reason, most contractors don't label very well.
As far as running phone lines over cat5 wiring goes, there is a standard mapping from the old solid-color conventions to the new "category class" striped wires. For the first four wires, these are Green (TIP of Line 1) becomes White with Blue bands Red (RING of Line 1) becomes Blue with White bands Black (TIP of Line 2) becomes White with Orange bands Yellow (RING of Line 2) becomes Orange with White bands There were actually colors for up to 25 pairs in a cable, and there are new striped-wire equivalents for all of them -- I've seen a better description, but one useful reference can be found at http://nps-vip.net/tester/colors.htm I believe there's a Wikipedia article which describes how the Category-class cables are used for Ethernet connections. I seem to remember that Ethernet really wants three twisted-pair lines, so if you want to run both Ethernet and phone to the same location you need at least eight conductors. (Homes which are wired with CAT5 often have a plug-in junction box somewhere which lets cables be more-or-less easily reassigned to different purposes. That does add to the cost, but preserves versitility.)
Cat5 can carry pretty well all the data you might ever want to move around the house. TV, audio, phone, internet whatever. You can bring a cable or four from every room down to a convenient closet, where you terminate all the wires.
As we continue through the 21st century, more and more appliances will use cat5 cables. Already we have PC's, gaming systems, cable TV and VOIP phones. Next in line will be refrigerators, ranges and even your pantry cabinets. As far as tying in your phone line goes. Why bother? Wired telephone is a 20th century concept. A thing of the past. A piece of history that is best forgotten. In the 21st century, the telephone is something you carry in your pocket.