How to Test a Residential Phone Jack Multimeter

telephone cable 1 image by Vonora from

The telephone company runs the wires in pairs (positive and negative) from its central office to the box called network interface device (NID) located just outside your home. From the NID, it is your responsibility to run another pair of standard colour wires to each jack on your wall where a phone will be plugged.

If any of the two wires inside your house would accidentally come in contact with each other, your phones will not work. Learn how to test your phone circuits and jacks using a multimeter to detect this problem and save yourself some cash.

Open the network interface device (NID) found on a wall just outside your home. Follow the wire running from the telegraph pole into your house to trace its location. Look inside the NID for a short wire with a modular plug at its end plugged into a jack. Hold the modular plug and unplug it from its jack to disconnect power from the phone company. Get a working telephone set and plug the modular connector found at the end of its patch cord into the jack.

Lift the phone handset and listen for a dial tone. The absence of a dial tone indicates a problem in the phone company's lines. Call the phone company to report the problem and wait for its repairman to fix the line.

Unplug any device, including telephones, from each telephone jack. Open a telephone wall jack by unscrewing the holding screw found in front of the faceplate, using a screwdriver. Go to the NID again and make sure that the modular plug mentioned in Step 1 is still unplugged. Phone lines carry a small amount of current that can cause non-fatal, low-voltage shock; unplugging the connector will allow you to work on the wires safely.

Look at the back of the face plate and view the exposed green and red terminal screws. Get a multimeter and set its dial to point to "continuity mode." Depending on the manufacturer, symbols around the dial of a multimeter may vary slightly from each other. Read the manual supplied with your multimeter to know the symbol representing a continuity test.

Hold the red probe and the black probe of your multimeter on each hand. Touch the green terminal screw with the metal tip of one probe, and touch the red terminal screw with the metal tip of the other probe, but do not allow the probes to come in contact with each other.

Look at the multimeter for a reading. The absence of any reading indicates that there is no short or contact between the two wires running throughout your phone lines. If the needles of your analogue multimeter move, or if there are any numbers displayed by your digital multimeter once you touch both terminal screws with the probes, then the two phone wires running throughout your home, or inside a jack, are coming in contact with each other somewhere along the line.