It can be fairly easy to run a second or third phone jack yourself if you don't want to snake through walls and you don't mind running wire along the baseboard moulding or using external mount phone jacks. This example is based on adding jacks to an existing live connection.
When terminating the connections it is best to install all the additional jacks and then connect the correct colors at the existing phone jack last. Pull the plug at the telco access box on the outside of your house before making your connections. This will protect the line from shorting and save you a possible ring voltage shock.
Use either standard phone wire, which consists of two pairs with the colors red/green and yellow/black, or category 3 or 5 wire. If you think that additional lines may be needed in the future, use the category 3 or 5 which consists of four pairs of wire.
This
photo shows the stripped end of a category 3 wire. The colors
are white/blue blue/white white/orange orange/white white/green
green/white white/brown brown/white.
(note: This means white with blue stripe and blue with white stripe etc.)
Start the wire from the existing phone jack leaving room for termination. Secure it neatly along the groove between the wall and baseboard using a special staple gun that has rounded staples for this size wire. You must use an installers (pilot) bit to drill through the wall.
This
photo shows a 1/4 inch installers drill bit.
The next step is to place the new jack. Mount the jack on the baseboard using double sided tape or screws. Strip the wire jacket back to expose the color coded wires inside.
Whether you use standard phone wire or category 3 or 5 cable, the important thing is to match the colors correctly on both ends. For example, if you connect a white/blue blue/white pair to the red and green terminals, make sure the added jack is wired straight through in order to maintain the line's polarity. A polarity tester is a tool that is helpful, inexpensive and available at any local electronics store.
This
photo shows a standard jack with four terminals: red, green,
yellow and black.
Strip each wire to expose about 1/2 inch of copper. Bend the bare copper in a small circle. Loosen the screw on the jack terminal, place the rounded copper end in place under the washer or grip and then tighten.
For a single phone you would connect the red and green wires to the red and green terminals in the jack. For a two line phone you would also connect the yellow and black wires to the yellow and black terminals.
If you need to place a single line phone and modem jack with separate numbers, you would install two jacks making sure that the yellow and black feed for the second line is connected to the red and green terminals on the second jack. The red and green terminals are what a modem or single line phone uses. Only a 2 line phone utilizes the yellow and black terminals in the jack.
Safety
Information from a "Leviton" kit 
