Checking Your Telephone Line




About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

One day you pick up the receiver for your land line phone and there is no dial tone. The line cord is plugged into the phone and the wall jack. You call the phone company to report your outage. The company representative asks if you have their insurance on the lines within your house. You respond that you do not. They tell you it could be very expensive if they come out and the problem is with the lines inside your house. Would it not be nice to know for certain the problem is with the phone company lines and not with the lines in your house?

There is a box on the side of your house, usually gray in color, where the phone lines come into your home. The one in the photo was painted over the last time the house was painted. For some reason, this house has two such boxes. The one on the left happens to be the active box for the phone service. The box may be mounted low on the house and may be a different shape.

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Step 1: Open the Terminal Box

This box opens with a 3/8 inch socket. Some open with a screwdriver.

Step 2: Check the Voltage

Inside the terminal box are three brass terminals. One will have a red wire. It is the terminal on the left. One will have a green wire. It is the terminal on the right. The third terminal is a ground wire connection.

Set your voltmeter for the range that can read 50 volts DC. Attach the leads to the terminals for the red and green wires. If all is working well with the phone company's lines, the meter will give a reading of about 50 volts, give or take a couple of volts. If there is a problem with the phone company's line the voltage reading will often be less than one volt. When the phone company representative tells you it will be very expensive for you if the problem is in your lines inside the house, just say, "I put my voltmeter on the terminals in the box where your lines enter my house. There should be about 50 volts DC at the terminals, but there was less than one volt." The representative's next words will be something like, "We will send someone right out."

Some newer terminal boxes have a modular plug receptacle rather than brass terminals you can access. Simply plug a phone into the modular plug receptacle and check for a dial tone.

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    12 Discussions


    2 years ago

    capacitors inside the phone box? Wow


    7 years ago on Introduction

    The pairs of wires you talk about are the ones that go to the successive phone jacks in your home. That way even if one outlet goes bad, wire break child experiment etc you will have service at other outlets. If you have only one jack inside and you have service outage to it check the wall plate for damage if its ok try another set of wires at the box, just attach them in the same config.

    J,R,D, Ltd

    8 years ago on Step 2

    Even on the newer terminal boxes you will probably be more likely to get them to send someone out on them if you check for voltage and recite the line above.

    2 replies
    Phil BJ,R,D, Ltd

    Reply 8 years ago on Introduction

    You are right. I used that line at least once, and it ended all of the malarkey. Thanks for looking and for commenting.

    J,R,D, LtdPhil B

    Reply 8 years ago on Step 2

    Your welcome, I wish I had found this instructable before we had trouble with our telephone line a couple months ago.


    10 years ago on Introduction

    the red and green line are usually the primary ones (Yellow and Black as secondary). Now depending on where you live, the main ones can also be white and blue for primary and white orange as secondary (OR more common, red and orange in older wire). Here is a common 25 pair color code: White / Red / Black / Yellow / Violet Blue Orange Green Brown Slate (Gray) So Pair 1 would be White+Blue and Pair 25 would be Violet+Slate (It runs by 5 and resets to next color) Ex: Black+Brown would be pair 14 Now in houses, you'll never see this expect on the telephone pole, but more commonly apartments have this color code. You still need a telephone meter to check for noise metallics, shorts, grounds etc... BUT a demarcation is suppose to be out a box outside your house OR to the first available jack inside. I believe it is law that the phone company has to provide dial tone to at least 1 jack even if the issue is other wiring/sets. Oh yeah Voltage can run from -46 to -54 volts and even higher when the phone is ringing. Hope this isn't too confusing.

    rc jedi

    11 years ago on Introduction

    that is what we call a "rubber duckie". we are required to replace them or extent to an accessible new "subscriber network interface" At the "plug"inside is the "demark" a point where federally regulated service ends and non regulated equipment begins. a test point. Company side of demark just contains a lightning arrestor. Nothing high tech. when you unplug the demark and plug your phone in, you are merely leaving the house wire off the circuit, and testing without your house wire. The red green code is an old code. New stuff uses white and blue.The white is the "tip, central office ground. Blue "ring" is same as red wire, -48v. When a phone rings, it jumps to 104v a/c 40 cycles a sec. That can sting a little, but only strong enough to surprise ya'. hee hee.

    1 reply
    rc jedirc jedi

    Reply 11 years ago on Introduction

    Your demark is looking crappy. but it is legal. subscribers have the right to do work anywhere beyond the demark. the wire coming to the house is phone company property, you may not modify it. It is federally regulated and you may be charged a fee or face an army of lawyers on retainer. But the stuff in your house is yours.


    11 years ago on Introduction

    In our box, there's a phone plug in. We can take our phone outside & plug it into that to see if we get a dial tone. Dunno if that's common or not.

    3 replies
    Phil BBrowncoat

    Reply 11 years ago on Introduction

    I mentioned in the last panel that some newer telephone boxes do have a modular jack inside. Those are relatively recent. A lot of buildings will have the older style boxes. The terminals inside are usually threaded brass posts with wires wrapped around them and a nut with washers to tighten over the wires. Radio Shack does sell a pigtail with a male modular jack on one end and four loose wires on the other end. The wires have crowsfeet terminals on the end of each. When I did not have a phone available, I used this pigtail and a voltmeter to test lines inside and outside the house. The box you describe can also open to reveal the wiring inside, I believe; but, the modular jack makes for a fast way to check for the proper voltage and a functioning line.

    Phil BBrowncoat

    Reply 11 years ago on Introduction

    It is OK. The first time I saw one of the newer boxes was a bit over 15 years ago. The phone company did some work in our neighborhood and replaced our older box with one like you describe. That was while we were living in another state. Here we have the older box in the photo. Still, it is a great feeling to tell the phone company representative you have checked and the problem is a dead phone company line. Sometimes there are extra pairs of wires. It was a big help to me to know the red and green wires are key (if you have a single line), and the voltage should be about 50 VDC.