How to Add a Phone Line

Connect the cable's red wire to the red screw terminal on the new jack. Don't connect the yellow or black wire.
Snap the cover plate onto the jack.

Now go back to the existing flush-mounted jack and find the unused (black and yellow) wires from the existing phone cable. Using wire connectors, join the yellow wire from the existing cable to the red wire of the new quad cable. Join the black wire to the new cable's green wire.

Replace the cover plate on the flush-mounted jack.