Depends. If you are using 100Base-T4 standards - that is you want to use cat 3, 4 or 5 cables ALL 4 PAIRS (8 wires) ARE REQUIRED. If you are using 100Base-TX (category 5 or 5e cables only) or 10Base-T the wires numbered 4 and 5 (one pair) and 7 and 8 (another pair) are not used in normal LAN operation but should still be connected since some equipment may use them for special purposes (we use them for power-over-ethernet in special applications). The spare pairs can be used for telephony - each pair will carry a single analog line so a cat 5/5e cable can carry 1 LAN (10 or 100 MB) and two telephone lines. However you are taking a RISK that in the future some genius may invent a fantastic application for one or more of these pairs and you may have to re-wire. Whether you consider this a high or low risk is a personal decision.
When wiring an analog telephone line on a LAN cable the following notes may help:
- All the LAN cabling MUST be category 5, 5e or higher.
- A normal analog phone uses 2 wires or one pair.
- If you are installing the cable and there is no EXISTING telephone connection on this cable use either spare PAIR of wires blue/blue-white OR brown/brown-white for each analog line.
- If the cable has an EXISTING telephone wiring connection find the other end of the cable and confirm that it is using either the blue/blue-white or brown/brown-white if so proceed as per 3 above, if NOT move the wiring to use either of these spare pairs (failure to do this will leave you with non-standard LAN wiring which will only lead to confusion in the future).