What UTP pairs do I use on a phone jack?

There you are with a four pair wire, and a six conductor phone jack that has terminations for only three of the pairs. Which pair gets left out? It helps to look at a list of pair designations:

Pair 1: blue and white
Pair 2: orange and white
Pair 3: green and white
Pair 4: brown and white

If you're wiring only three pairs, you use the first three. If you're wiring only two pairs, you use the first two. That's easy enough. But sometimes things get more confusing and you have to terminate quad wire onto a Cat 3 jack or attach Cat 3 or Cat 5e cable to a telephony plate with quad wire screw terminals. In those cases you need to know how the quad wire conductors and Cat 3 or Cat 5e cable conductors match up so that you can translate between the coloring schemes. You may run into "quad wire" that has six conductors; the additional two conductors (blue and white) are included in the list below:

Red: blue conductor of the blue and white pair
Green: white conductor of the blue and white pair
Yellow: orange conductor of the orange and white pair
Black: white conductor of the orange and white pair
Blue: green conductor of the green and white pair
White: white conductor of the green and white pair