First the disclaimer:
If you are a purist, one that think that every rule must be followed, do not read.
If you are interested in testting “what is posible”, the answer is as follows:
You may run data using only 4 wires out of the eight that normally are on the RJ-45.
That means that are four wires on the cable that do not get used.
Those wires are there to balance the capacitance of the cable, in such a way that if you need very high rate of data and the cable is close to 300 feet long (100 meters), you still have the full category 5 specs.
But if you are the regular Joe, that have a normal need, those wires could be used for:
1) A second ethernet conection. You open the eight wire cable, connect four wires in one RJ45, and the remaining four wires in a second RJ45.
You end up with two ethernet segments, or lines, independients of each other.
2) Or what will make many people roll their eyes. Use the remaining four wires to run phone lines trought the house.
The fact is that when someone is ringing the line, will be around 100 volts A.C. in a pair adjacent to the ones carring the data .
Most likely it will degrade some the performance of the data link during the ringing cycle of the phone.
During normal usage of the phone, I think that you will be hard pressed to see any diference.
Now if you do not like the answer, i am sorry. I know for a fact that it works most of the time.
is not waht I do when I could run a new cable, but on some buildings that is not posible.