Telephone lines are made of two copper wires that run from the telephone company's central office into the network interface device (NID) mounted on a wall just outside your home. The NID serves as a demarcation point where the responsibility of the telephone company ends, and your responsibility as subscriber begins. The NID also serves as terminals where you can connect and disconnect from the telephone company lines. Although not commonly done, you can also disconnect a phone line from inside your home.
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Locate the network interface device (NID) your telephone service provider installed on a wall just outside your home. Follow the wire that runs from the NID into your house until you are able to locate the telephone junction box — a device or small plastic box that contains terminal screws. The cable coming from the NID runs into the junction box, where it is distributed to each phone jack inside your home.
Unscrew the cover of the junction box and open it, using a screwdriver. Hold the cable coming from the NID, and look at each color-coded wire coming out of the cable. Each wire is connected to a terminal screw on the junction box.
Loosen the terminal screws that hold each wire, using a screwdriver, and disconnect each wire from its terminal. Pull the cable away from the junction box once all the wires are disconnected. Cut off the tip of each wire to remove any exposed copper wire, using diagonal pliers. Wrap electrical tape around the tip of each wire. Replace the cover of the junction box, and listen for any sound. Disconnecting the main line from the junction box cuts off any power or signal to all your telephones.
Disconnect power and signals from the telephone jack in case your telephone line does not have a junction box. Open the faceplate of your wall jack, using a screwdriver. Loosen the red and green terminal screws behind the faceplate and remove the wires attached to the screws, using needle nose pliers. Wrap each bare wire individually with electrical tape. Tuck the wires into the telephone outlet box and replace the faceplate onto the outlet box with a screwdriver.