Cat 5e 2 pair is usually used for data communications specifically ethernet and token ring. These data networks usually use one pair for data receive and the other pair for data transmit. I have seen this type of cable come in 2 pair and 4 pair. The pairs are twisted together tightly to prevent crosstalk interference between pairs. Usually for data-comm infrastructure 4-pair cable is used since you need a minimum of 2 pairs if something goes wrong with one pair of wires you still have 2 spares to fall back on, before you have to re-pull the run. 2-pair cable can be pulled to save money but in the long run it could be risky.
Since you are using only one pair this is most probably used for analog voice communications of some sort.
Never saw cat 5e in two pair. I have seen cat 5- 2 pair used for patch cords. I didn't think it was possible to maintain the cat 5e characteristics without the other two pair.
You can use it for 10/100 baseT ethernet as only two pairs are used but you will have problems trying to figure out how to terminate it.
Since you are only using this for voice it's no big deal and that's probably about all it's good for. If you are looking for cost effectiveness look at cat 3 for voice but I suspect somebody was trying to unload that stuff real cheap.
Do not think you could even call 2-pair Cat5E. Couldn't be used for 1000Base T systems at all (4-pair technology). Certainly does not meet TIA/EIA-568A-5 standard for Cat5E. Should be OK for POTS, but I wouldn't trust it for ethernet.
Edited spelling errors
[This message has been edited by dereckbc (edited 11-11-2004).]
Your Cat 5 CAN be used for 10/100M Ethernet- The standard refers to the number of twists per inch, and wire gauge. We used to use this when wiring 10M building networks. I have this in my house for my 100M network.