A 4 wire cable has the red and green as one pair and yellow and black as the other.
One wire of the pair is referred to as the tip and the other wire of the pair as ring.
When swapping wires, etc., the yellow corresponds to the red and black corresponds to the green.
If you have the standard rj jacks then all four wires should be connected.
You can switch them or buy a 3 way connector that has a plug in for line 1, lines1&2, and line 2 and then plug into line 2 for the second line.
Like I did, you can buy the blank plugs and a plastic plier like device at the electronic store and make your own extension cords from phone/computer to jack.
Then you make your own converter by switching the wires in the plug before crimping.
If you have a jack that uses both lines such as for a two line phone, watch out for some of the extension lines supplied with phones/modems cause some of them save money by only using 2 wires instead of 4 and the cord looks the same but if you look at the clear plastic plug, you will see if there are 2 or 4 wires.
Also remember in making your own extension lines with a plug on both ends that the order for putting the line into the plug is reversed on the other one.
[This message has been edited by Joe Tedesco (edited October 03, 2002).]