If you go looking for a phone jack in an older building, you might be in for an unpleasant surprise. In the past, phones were wired directly to the lines at connecting blocks. Unless they've been updated, all you'll see is that plain square block with a vacant hole in one side, or perhaps just a cut wire. Today, land-line-connected phones, answering machines and other devices use modular plugs and jacks, making it possible to move them from place to place. With a new jack and some simple tools, you can add that modern convenience to your old building, too.
Mount a new modular phone jack in a suitable location. Make sure the jack is within reach of the old phone line, and that you will have enough free line to wire the new connections.
Strip 3 inches of the outside sheathing from the old phone cable with a sharp knife. Be careful not to cut the small wires inside the sheath. Unless your phone line is very old, there should be four wires. Separate the wires so that you can work with each one without damaging the others.
Strip 3/4 inch of insulation from each of the four wires using a small gauge wire stripper or sharp knife. Be careful not to damage the wires.
Remove the cover on the modular jack. Loosen the four colour coded screws. Form a small, counterclockwise loop with each of the wires around the screw that corresponds to the wire colour. Single line service only requires two wires, usually green and red. Most older phone cable will have a yellow and black wire, as well. Connect all four wires to provide for future updates. Work carefully; phone wires are very thin and old wires break easily.
Tighten the colour coded screws, being careful to route all the wires clear of each screw as you do so. Replace the cover on the phone jack. Plug in the phone and test the line.
Some phone jacks have connectors that don't require stripping the old wires, which makes the installation process much easier. If you are going to run a line to another jack, install that line at the same time to minimise handling of the old phone wires. Very old phone lines might have fewer wires or not use the conventional colour coding. In that case, you will have to figure out which wires are active by tracing them from the phone company network interface device somewhere on your side of the phone line service entrance.
Observe all electrical wiring precautions when working on phone lines.