Jim:
Though it's probably done frequently, I never recommend that one wiring system be installed that can alternate between voice and data. We always provide CAT 5e (currently) for the network and CAT 3 for voice. The CAT 5 is terminated on 8 pin 568B jacks while the CAT 3 is terminated on 6 position 4 pin USOC jacks. The first two pairs are normally punched down, the other two are spares. The spares can be used for faxes, modems or even another phone. The additional jacks are added to the plate as necessary. The reasons for all of this are:
1) CAT 3 plenum is much cheaper than CAT 5e plenum.
2) CAT 3 is easier to terminate because you don't have to untwist tightly twisted pairs.
3) CAT 3 is easier to "tone out".
4) The network and telephone system are two completely different animals most often maintained and administered by different people.
5) The network wiring gets terminated on a patch panel which in turn is connected to a hub or router with patch cords.
6) The voice wiring gets terminated on a 66 or 110 block. From there the first two pairs are cross connected to a similar block from the key system or PBX. The spare pairs, where used, are cross connected to whatever they need to be connected to. You can't do this with a patch panel.
7) Most voice terminals (telephones) use the familiar 4 pin 6 position plug. While they will fit into an 8 position jack, the sides of the smaller plug can damage the two outside contacts on the jack.
8) Voice terminals use USOC jacks. The two pairs are the normal w/bl and w/or which are pairs 1 and 2. 568B data jacks use w/bl and w/gn as the corresponding pairs if you were to use it for voice. Very confusing especially since they are punched down in order of 1, 2, 3, 4 at the block.
