How to Troubleshoot Home Telephone Wiring

Home telephone wiring is your responsibility as a homeowner. The phone company is only required to provide a working telephone signal to your Network Interface Device (NID), which is the box on the outside of your house where the phone lines enter from the street.

Finding a problem inside your house can be challenging. Troubleshooting is complicated by the lack of wiring diagrams of most household installations. Finding the problem may require a bit of trial and error.

Verify good phone service

  1. Locate the NID box outside your house. It is typically attached to your house with wires running into it, either from underground or from a utility pole.

  2. Remove the screws and open the NID box.

    The NID is your property, and is designed to be accessed by the homeowner to troubleshoot phone service. You do not need the phone company's permission to enter this box.

  3. Examine the interior of the box. There should be one or more RJ type connectors plugged into sockets. Remove each of these and plug a corded telephone into the socket.

    If your phone service is working, you will hear a dial tone and be able to dial calls from the corded phone plugged into the NID.

    If there is no dial tone, the problem is with the telephone company. Contact them and inform them you have no service to your NID. It is their problem to fix.

    If there is a dial tone, your phone service is operating correctly, and the problem is in your wiring. Reinsert the modular plugs into the socket and proceed to the next section.

    Don't bother reinstalling the cover to the NID box, you may need access again.

Remove all telephone devices

  1. Disconnect all telephone and telephone devices by removing the jacks from the wall outlets. Do not overlook wall phones, fax machines, computer modems, answering machines, video recorders, or satellite and cable boxes that connect to phone lines.

    A malfunctioning device can cause impair all the other devices in your house.

  2. Identify the phone jack in your house that is physically closest to the NID. If you are unsure, take your best guess.

  3. Connect your corded telephone into the jack identified in step two and check for a dial tone.

    If the corded phone functions properly, the problem is in one of your other devices. Plug them back in one at a time until the offending component can be identified.

    If the corded phone does not function, proceed to the next section.

Troubleshooting wiring

  1. Remove all telephone equipment from your phone jacks, and disconnect the jacks in your NID.

  2. Remove the wall plate from the modular jack closest to your NID. If you are not sure which is closest, take your best guess.

  3. Place one lead of your ohm meter to each of the red and green wires or their associated terminals. The meter should register very high resistance between these terminals. On an analog meter, the needle should barely move.

    If the meter indicates a short, or low impedance conditions, the red and green wires are touching somewhere in your wiring. Proceed to the section titled "Finding a Short Circuited Wiring Pair."

    If the meter does not indicate a short, proceed to the section titled "Troubleshooting an Open Wire."

    Your primary phone line is carried on the red and green wires. The black and yellow pair is for a second phone line.

Finding a short circuited wiring pair

  1. Remove each phone jack from the wall by removing the mounting screws.

  2. Hold the modular jack with the opening down and blow compressed air into the jack. You are attempting to remove any stray wire or conductive objects.

  3. Examine the screw terminals and wiring on the rear of the jack. You are looking for any signs of the red and green wires touching.

    Repair or remove any sign of shorted red and green wires.

    Keep rechecking the resistance between the red and green wires or terminals. When the resistance becomes very high, you have fixed the problem.

    If the zero resistance between red and green terminals remains, the problem is in one of the wire runs. Step four begins the procedure to locate this short.

  4. Disconnect the red and green wires from every phone jack. Break any connections involving red and green wires found in the boxes behind the jacks.

    If there are wires which are wrapped around a screw without being cut into separate pieces, cut these at the point where they are joined to the screw.

  5. Measure the resistance between all the red and green pairs of each conductor in every box. One wire in two different boxes should show zero resistance. This is the wire with a short somewhere along its length.

    There is no easy way to electrically determine the exact spot on the wire where the short is present. This entire wire will likely need to be replaced.

    If you do not detect any wire with a short between red and green conductors, begin reconnecting all the wires back to the appropriate terminals. Check for the presence of a short after each connection.

    It is possible for the short to have disappeared as a result of your manipulation of the wires.

Troubleshooting an open wire

  1. Draw a household telephone wiring diagram on a sheet of paper showing one box for every phone jack in your home, and a block to represent the NID.

    Double check that the connectors in the NID are disconnected before proceeding. The phone company may detect some of the temporary connections you will be making unless you disconnect your house from their network.

  2. Disconnect every red and green pair from the terminals in all the jacks.

    If there are wires which are wrapped around a screw without being cut into separate pieces, cut these at the point where they are joined to the screw.

  3. Choose a red and green pair in the wiring box you believe is closest to the NID.

  4. Connect the red and green conductors together. This is only a temporary connection, so a quick pigtail will work fine.

    Mark this wire with a small piece of masking tape with the number "1" written on it.

  5. Use your ohmmeter to find a red and green pair in another box which show zero ohms between them.

    Mark this wire with a small piece of masking tape with the number "1" written on it.

    Annotate your telephone wiring diagram from step one to show the locations connected by the wire marked "1".

    Remove the temporary pigtail.

  6. Repeat the procedure in step five with all the other wires.

    Remember that one wire will connect to the NID. The red and green wires are the middle two of the phone jack in the NID, so verify continuity to these points.

    When you have tested every wire, you should end up one wire in two different boxes that does not appear to be connected. This is the wire with the open conductor. Replace this wire.


  • Make simple wiring diagrams of every box as you disconnect existing wiring. This will simplify reconnecting wires later.
  • In the United States, interior wiring is the homeowner's responsibility and the NID is provided for customer access. Rules and responsibilities may be different in other countries, so check with your local provider or regulator.

About the Author

Andrew Hazleton has been writing on a freelance basis for more than 20 years, and his work has appeared in national, regional and in-house publications. His work has appeared in "Sports Illustrated," "IEEE Spectrum," "Popular Photography" and several newspapers. Hazleton has a Bachelor of Science in engineering from Lehigh University and a master's degree in management from Pepperdine University.