How To Connect A Second Phone LIne

by Jeff Fisher

One of the most common questions we get asked goes something like this:

We had the phone company install a second phone line, which we're going to use for our modem. But they wanted $200 to connect the line to an inside jack, which we though was outrageous and declined. So now they say the line is "installed." How do we hook it up to use it inside?

This application note will attempt to take any novice through the process. (Or at least give you some pointers that will help.)

The Theory

1242a.jpg A phone line uses two conductors. Typical old-style telephone cable has four conductors. (Red, Green, Black, and Yellow or White.) The first phone line will be on the Red and Green wires. The black and yellow wires will be unused and may not even be connected. If you connect them right, you can use the black and yellow wires to carry the new line from the outside of the house to the desired wall-jack(s). Standard modular wall-jacks can support both these lines. And you can use either a two-line phone or a plug-in two line splitter to get to the second line.

Note: Sharing two phone lines in non-twisted "quad" telephone cable will probably introduce some crosstalk. You may be able to hear whatever is going on the other line at a very low level. There isn't anything you can do about this except rewire all your phones with more current twisted-pair wire.

Telephonic Archeology

The first step is to perform a bit of what we call "Telephonic Archeology" by examining your present wiring to try to determine what goes where.

hdi11e.jpg Go around the house with a small straight screwdriver and "pull" all the phone jacks. For surface mount boxes, remove the single screw and gently pull the cover away. For jacks that are built-in to the wall, remove the two visible screws and gently pull the wall plate away from the wall until you can see how the wires are connected behind the plate.

If you are being watched, stare intently at all these wires, rub your chin, and nod knowingly (this is very important). You may do this for an extended period if you wish. Congratulations...you've just done exactly what the telephone installer gets paid $200 for.

You will most likely see one cable going to the jack, either from inside the wall, or stapled to the surface of the wall. This cable will probably have four solid conductor wires in it; one red, one green, one black, and one yellow or white. If the wire doesn't have four conductors, you have a non-standard installation and you'll need help. This cable either goes straight to the DMARC block (more on this later) or it connects to the back of another jack...which does have a cable that goes back to the DMARC block.

If there is more than one cable, and the wires from one cable are tied to the same color wires from the other cable, (especially the red and green) then you are looking at a "daisy-chain" jack. One of the cables probably goes back to the DMARC block, and the other(s) go to other jacks. (It doesn't really matter at this point.)

Each jack should have 2 or 4 screw terminals. They may be labeled with the wire color: (R) red (G) green (Y) yellow (B) black.

The jacks will also have 2 or 4 individual wires that run from the modular jack to the screw terminals.

The main thing you're trying to determine at this point is whether all jacks have all four wires, and if they are connected. Leave the jacks loose, you may need to work on them later.

hdi11b.jpg Now you need to find your DMARC block. An explanation: The phone company has this concept of a "point of demarcation" where they are completely responsible for the phone lines to a certain point...and you are responsible for them from there on. This point is usually on an outside wall of a house, or possibly just inside a garage. Usually they are a little gray box with something about the phone company written on them and may have some rather large wires going into them.

hdi11a.jpg Newer DMARC blocks have a divided interior with one sealed by a special bolt and the other side user-accessible with a standard screwdriver.

When the phone company added your second line, they probably added it to your existing DMARC box. But they could also have replace your existing box with a new version that supports two (or more) lines, or they may have added a second DMARC box for the new line. (It's up to them which they do.)

Open up the "user serviceable" portion of the DEMARC block. There will be two or more screw terminals inside. Possibly with a colored label or R/G/Y/B labeling. Note the cables that come from the house and which wires connect to which terminals. We'll figure these out later.

Get Ready

Now ask yourself what you want to do with this new phone line.

Probably the easiest thing to do is to connect the new phone line to all (or most) jacks. This new line will be "line 2" in your jacks. Any 2-line phone plugged into these jacks will allow access to the second line. Or you can use a plug-in two line splitter to connect a phone/fax/modem directly into line two. To do this you just need to make sure that all 4 wires are connected in each jack, and get the black and yellow/white wires connected to the proper place in your DMARC box.

If it's a fax or modem line, you may need to only to go to a certain jack. To do this, you'll need to get the black and yellow or white wires hooked up in the jack, and figure out where the cable goes and which cable it is back at the DMARC box. If the cable goes straight to the DMARC box, you're in business. Just hook up the wires to the new line (more later.) But if the cable goes to one or more other jacks first, you'll have to make sure that the black and yellow/white wires are connected "passed through" all the jacks between the DMARC box and the jack where you want the new line.

Get Set (Handset...get it?)

hdi11f.jpg OK. Now it's time to actually do something. You'll need a bit of test equipment:

  1. A working telephone.
  2. Two phone cords with a modular plug at both ends.
  3. A plug-in two line splitter. (Which you can get from us or just about any office supply store or Radio Shack.) This device plugs into a 2-line jack and splits line 1 to one jack and line 2 to the other jack.
  4. A medium sized straight screwdriver and a pair of pliers.

hdi11g.jpg Take one of the modular phone cords and cut off one end. Strip off about three inches of the cable jacket. If the cable has four conductors, cut off the outer two. Strip the remaining two conductors. If the wires are colored, these will be green and red.

What you have done is build home-brew versions of sophisticated test equipment. If you plan on making a career out of this, you can buy the real thing.


hdi11c.jpg Plug the cable with the stripped wires into the telephone and take it out to the DMARC box. Inside this box, you should see some green and red wires going to two screw terminals. If you connect one wire from your improvised "butt-set" to each of these screw terminal, you should get a dial-tone when you pick up the phone. To test to see if you have the "correct" line, dial the number. (Make sure nobody is using the phone inside.) If you get a busy signal, you are probably on that line. (This method isn't fool-proof, but the special number the phone repairmen use is apparently an industry secret, to which we mere mortals are not to be trusted.)

If there is a modular plug and jack near these terminals, this is your "test port." You can unplug it (which will disconnect your phones) and plug a phone directly into the jack to connect to the line. The phone company will tell you to do this to test the line before they come out to fix anything. It proves that the problem is either on your side of the wiring or their side. If you don't have a modular plug and jack, don't worry, you don't need them for this exercise.

Determine which cable feeds which jack(s) inside your home. (You may not need to do this if you are going to feed the new line to all jacks.) Plug the standard modular cable into the telephone. Disconnect one green wire from the screw terminal. Now run around the house plugging in your telephone to see if you have a dial-tone at each jack. The jack(s) that stopped working are connected to the wire you just disconnected. Back at the DMARC block, label the cable with the jacks that it serves. Reconnect the green wire. Disconnect another green wire...repeat the above until you have identified all cables. (You may find that some cables don't appear to go anywhere. That's not unusual in an older home.)

hdi11d.jpg Determine which terminals in the DMARC block are the new line. Connect your makeshift butt-set to the other terminals in the DMARC block. They may be color coded black and yellow, or a second set of green and red, or have B and R labels, or simply be two more "posts" opposite the ones in use. Connect to terminals until you get a dial-tone and call the new number to make sure you get a busy signal. (You can also go inside and call the new number to see if the test phone rings.) When you identify these terminals, connect the black and yellow (or white) wires of the jack(s) where you want the new line.

You're almost done. The only thing left is to make sure the black and yellow wires are carried all the way through and are connected in the target jack. Plug your test phone into Line 2 of the two-line adapter and plug the adapter into the target jack. If you get a dial-tone...you're in business! If not, go around to any other jacks that are on that cable and make sure that the black and yellow wires are "passed on," either by splicing the two blacks together and the two yellows together, or by connecting both blacks to the B terminal and both yellows to the Y terminal in the jack.

If you want all jacks to have the new line. Just make sure that all the jacks have the black and yellow wire connected.

Now you can go around and button up all your jacks and the DMARC block. You should have a good idea of how your phones are wired, and how they connect to the DMARC block. If you ever have phone trouble in the future...you'll be one-up on most people!

That's it for now...Enjoy!


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