if you use 568b, any single pair mounting cord/device will be connected to white/blue pair (the two center pins) 568a makes the two center pins on the voice station connecting block into the white/green pair. In most voice systems, documentation says connect the basic two wire station to white/blue. also note that rj11-rj12 starts with the first pair being the two center jack pins i.e. blue/white or red/green
