To install a telephone jack, use the combination tool or wire strippers to strip the outer sheathing from the telephone line. Most of the newer phone cables have a pull cord inside the cable that strips the sheathing as you pull on it. In this case, strip only a small amount of the sheathing using your strippers and use this technique.
Otherwise, carefully place the stripping tool cutter potion of the pliers over the cable to score the outer sheathing. A four-wire cable will use the 10 gauge cutting hole. Twist the stripper handles to make the cutters cut through the outer sheath. Be careful not to cut into the insulated wires within the cable.
Pull the sheathing off of the end of the wire and untangle the wires. Strip the individual wires using the 20-22 gauge slot on the stripper tool. Strip about 1/2 to 3/4" of insulation from the wires. In most cases, you'll only be needing the red and green wires to connect the phone line. There is a possibility that the yellow and black wires are being used for a second line for a separate phone line for another number or fax line.