How to Test a Phone Line with an Analog Voltmeter

If you don't have a phone line tester, an analog voltmeter can be used to to perform some rudimentary tests on the telephone line. This handy step by step guide will show you how.

(Note - If you are having a problem with a phone line, try switching out phones to make sure a bad phone is not the culprit. This should be the first thing you try whenever you can't get a dial tone or the tone is intermittent.)

Testing to See if Your Lines are Connected to the Telephone Company

Step 1 - Locate Problem Line at the Main Junction Box

First, go the network interface box that is mounted somewhere on the outside of your home and locate the line wiring inside the box.

Step 2 - Test Incoming Telephone Line Voltage

In order to check if you are connected to the telephone company, you don't need to unplug anything or loosen any wire screws. Instead, set your voltmeter to "volts" or "VDC" and touch the black probe lead to the red wire inside the box. Likewise, touch the red probe lead to the green wire.

Your voltmeter should display a low voltage reading of around 45 to 48 mV. If it does, then your lines are properly connected to the telephone company. If there is no voltage reading displayed, or if it is zero, then your lines are not getting a signal from the telephone company.

Testing Internal Wiring for Continuity

Step 1 - Disconnect Wiring in Main Junction Box

Go the network interface box outside your home and disconnect the plug (inside the outside junction box) for the line that is not working. If you only have one line, there should be only one plug that has colored wires attached to it. Remove the short plug wire. Afterwards, you are no longer connected to the telephone company and can test your internal wiring for continuity.

Step 2 - Set Your Voltmeter to Test Continuity

Follow the directions that came with your voltmeter for setting it to test for continuity or an open circuit.

Step 3 - Disconnect All Phones

Removes the line plugs from all phones that share the line.

Step 4 - Understanding Continuity

If wired correctly, each colored wire in the line should be totally independent of the others. So, none of the colored wires should ever touch each other. You can use the continuity test function of your voltmeter to confirm that no two wires are coming into contact with each other.

A wire that has 'continuity' will allow electricity to flow freely through the length of the wire without obstruction. However, two bare wires that are touching each other also have continuity. Since the wires should never touch, continuity indicates a problem in this case.

Step 5 - Open a Terminal Box

Use the screwdriver to open a terminal plug box (where you plug-in your phone).

Step 6 - Test the Wiring

Place the leads on any two wires. If the voltmeter indicates that there is continuity, you have a problem with your home's internal wiring and will need to repair it.