Introduction: Retrofit a PBX to Existing Phone Lines

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Due to the proliferation of cellular networks, most homes have abandoned their land line telephone plan. The infrastructure is still there though, hidden in our walls. Why let all that copper go to waste? Let's talk about how inexpensive and easy it is to reclaim this technology, making it useful again in today's internet connected world.

This time, we're going to talk about how to take back all those unused telephone lines hiding in your walls. This is much easier than you might think, and doesn't cost a thing. As a bonus, there are more lines in there than were ever actually connected to anything!

Follow along as we 'connect all the lines', building our own private branch exchange telephone system right on our house's old phone lines.

For a much more detailed look into what a PBX is and what it can do for you, check out our recently featured Instructable Cheap and Easy Guide to Building a Private Telephone System (opens in a new tab).

Step 1: Understanding How It Works

A telephone requires two wires (a pair) between its jack and the outside world (or a PBX extension jack, in our case). These two wires are called Tip and Ring. The terms Tip and Ring come from the days when telephones were manually switched by an operator who would plug the caller's line into a switchboard using cables terminated with 1/4" plugs. A 'TRS' 1/4" plug has three connection points; Tip, Ring and Shield. You'll see this same TRS configuration (only smaller) on any pair of standard headphones.

In a modular telephone cable there are multiple wires, these are color coded green for Tip and red for Ring. You may have noticed that modular cables often have four wires (the other two generally being black and yellow). The other two wires are a second pair where black is Tip and yellow is Ring. So a modular jack / cable with four wires supports two separate phone lines!

When you open up a modular jack on the wall, things get a little more complicated (but not really).

It is quite possible that you'll find that the wire snaking through the wall to the back of the jack includes four wires; green, red, black, and yellow. You'll also probably notice that there are four screw terminals (or similar connection mechanism) labeled G R B and Y. Really easy! This type of cable supports two phone lines.

If you open up a jack there is a chance, however, that you'll find something a bit more puzzling (but not really).

Another popular type of cable for use in telecommunications is a '25-pair color code' cable assembly. These do not use our old friends GRB&Y, but are actually even easier to understand once you know the trick. The assembly is a collection of color coded pairs (which you may have guessed from the name). Each pair has a major color and a minor color. The Tip in each pair is mate major with a minor tracer, while the Ring in each pair is mate minor with a major tracer. Generally in telecommunications you will only see up to the first five pairs in a cable assembly.

They are:

  1. White / Blue
  2. White / Orange
  3. White / Green
  4. White / Brown
  5. White / Slate

In our house, we found lots of this type of cable. Our's happened to be three pair, and the manufacturer didn't include the tracers on the ring wires. So the majority of our cable was:

  • Pair 1: Tip White with Blue stripes, Ring Blue
  • Pair 2: Tip White with Orange stripes, Ring Orange
  • Pair 3: Tip White with Green stripes, Ring Green

Now that we know what all the colors say, let's talk about where the wires go! There are two typical methods used for wiring up telephones in a house; Star and Loop.

In a Star system, there is a cable running from every modular wall plate to a central location. At the center is the network interface, where things come together. For example, all of the Pair 1 Tips are connected together and to the Telephone Company's Tip, while all of the Pair 1 Rings are connected together and to the Telephone Company's Ring.

In a Loop system, there is a single cable looping through the whole house. On one end the Tip and Ring of a pair are connected through the network interface to the Telephone Company's Tip and Ring. On the other end is the last modular jack in the loop.

In either setup, the network interface is the system's connection to power and functionality. Those of you old enough will remember that even when the power went out, old telephones still worked. This is due to the fact that landlines are connected to a battery at the phone company end! They apply -48 volts of direct current potential on the Ring and no potential on the Tip. When a call comes in, the phone company adds a 20Hz 90 volt alternating current signal across Tip and Ring, which causes your phone(s) to ring.

Armed with what we know now, let's open up some modular jacks and see what kind of system we are dealing with.

Step 2: Topological Archaeology

Our house is three stories, built in the 1970s, with a few additions and changes over the years. As such, it was really important for us to open up and check every single modular jack in the house before we changed anything, or connected anything to our existing telephone system.

For context, the ground floor of our home is the local makerspace 'The Rabbit Hole', the first floor is typical residential, and the second floor is Tymkrs (our recording studio, server room, offices, electronics lab, and main set). I will include [codes] that correspond to the green location codes pictured in the white board map of the system.

The Network Interface

We don't have a network interface box. Turns out the phone company's lines come inside directly off the telephone pole through a wall to a lightning arrester / terminal block under the breaker box in the Rabbit Hole's shop [S]. This says to me that this is original to the house's construction (because it's been a very long time since things were done this way).

The Mystery Cover

Up in the second floor office there is a mysterious jack plate displaying a Bell System logo blank instead of a modular jack [AD]. Curiouser and curiouser.

The Jacks

  • Ground Floor Lounge [SSK]
  • First Floor Hallway [H]
  • First Floor Master Bedroom [MB]
  • First Floor Guest Room [GB]
  • Second Floor Electronics Bench [B]

Once we located and named all of our modular jacks, we used a flat head screw driver and opened them all up.


Where the phone lines come into the house, we found a three pair cable with only the WhiteBlue/Blue pair connected to the telephone company's interface. We left the lines from the telephone company connected to the lightning arrester and disconnected the WhiteBlue/Blue pair from the terminals. All the phone lines in the house were now dead (and safe to play with!).


This is a stainless steel Bell System jack with tabs for hanging a wall mounted telephone onto it. Inside it had a punch down block terminal. The cable running to it was three pairs ending here, with WhiteBlue/Blue connected to Tip/Ring on the punch down block. Being that the cable ended here, we initially thought we were dealing with a Star system.


This is a round plastic Bell System jack. Inside it had GRB&Y screw terminals. The cable running past it was three pairs, with WhiteBlue/Blue connected to the Tip/Ring (G/R) terminals. Being that the cable left this jack in two directions, we knew that this wasn't a Star system, it must be a Loop system with [SSK] being the end of the loop.


This is a little box stuck to the baseboard in the master bedroom with a stapled wire running to it along the baseboard from a a hole in the wall. The other side of this wall is the guest bedroom [GB]. This was much more modern, and obviously added on decades after the rest of the system was put in place. I didn't even have to open it to know that it was a two pair cable GRB&Y. Cheap, and tacky.


This is a round plastic Bell System cover with a round hole in the middle with GRB&Y screw terminals and out of which a modular cable hangs loose. Inside, the cable running past it is three pair with WhiteBlue/Blue connected to Tip/Ring (G/R) terminals. Green/Red of the modular cable are connected to the G/R terminals, and Black/Yellow of the modular are also connected to the G/R terminals. There is a two pair GRB&Y cable here as well, with G&R connected to the G/R terminals (its black and yellow lines are left hanging loose, crappy job someone). This is obviously the wire over to [MB].


This is a slightly newer looking round plastic Bell System modular jack with GRB&Y screw terminals. Inside, a cable ends here. The cable is two pair; G&R connected to the G&R terminals, and B&Y connected to the B&Y terminals. This is the only jack in the house wired up for dual lines. Huh ... this is quality work, but obviously not part of the original loop. What is going on here?


This is a painted over round plastic Bell System cover. Inside, a three pair cable passes by. There is also a newer GRB&Y cable ending here with G connected to WhiteBlue, R connected to Blue, Black connected to OrangeWhite, and Yellow connected to Orange. Ah, this must be where [B] connects into the system!

Deductive Reasoning

Now that we've taken a look at everything. It would seem that we have a loop system with two branches: [GB] and [B]. It would also seem that every single modular jack is wired (some in round-about ways) so that its Red/Green pair is connected to the WhiteBlue/Blue pair in our loop.

Once you collect all of the facts (and armed with your knowledge of how it works), it is time to draw a map of how you think everything is connected. I drew mine on a whiteboard, leaving lots of space for notes and adjustments later.

Step 3: A Plan

Kitting Up

At this point, we've learned a lot just wandering around the house with a flat head screw driver looking at colors. To cook up a real plan to use these wires for our own purposes, however, we're going to need to up our game. For this, I grabbed a tool box and filled it up with a few toys.

  • PBX
  • Wire Strippers
  • Solder
  • Heat Shrink
  • Soldering Station
  • Multimeter
  • Work Light
  • Basic Telephone
  • Helping Hands
  • Hemostat
  • Flathead Screw Driver
  • Philip's Screw Driver
  • Modular cable with stripped Green & Red leads

Not all of this is absolutely required, but if I am going to make changes to any wiring in our house, I am going to leave it in a much better state than in which I found it. While crimp connectors and loose wires may have been good enough for the telephone company tech, I want my patches to be properly soldered and sealed up in heat shrink tubing.

Initial Testing

Once you have a tool kit put together, it is time to test any assumptions you asserted in the previous step. In our house this is pretty obvious - everything connects to a single pair. So I head down to [S] and connect the WhiteBlue/Blue pair up to Green/Red on my stripped modular cord, and plug that into an extension line on my PBX. I power up the PBX. At this point every modular jack in the house should be connected to that live PBX EXT.

I go to each jack, plug in my basic telephone and check for a PBX dial tone. It looks like our footwork paid off. Every single phone jack in the house has a dial tone!

If you find that what you thought would happen didn't, then it's time to go back to the previous step and do a bit more archaeology. The more you know for certain about the existing system in advance, the easier it will be to formulate a solid plan for how to rewire it.

Where to put the PBX

The PBX is where all of your lines come together. If you have a Star system, the PBX naturally goes in the center of the star. If you have a Loop system, you might be tempted to put the PBX at the beginning of the Loop. Be aware that if you do this, you will only have as many EXT lines as you have pairs. If you think about it, by placing your PBX somewhere along the middle of the loop (and cutting your loop in half) gives you twice as many discrete pairs to work with. Another thing to consider is that if you place your PBX close to some of the equipment it has to connect to, those devices don't need to take up pairs inside the wall at all.

In my case, I noticed that [AD] is in the middle of the loop (doubling my pairs) and at a branch (giving me two more pair) and right above one of the desks in the office (saving me a pair for one of the phones) and close to the server room (saving me two more pairs for equipment that needs to be close to the network switch). Thank you mystery cover! By being smart about where I'm putting my PBX, even though my Loop system only has three pairs, I'm going to be able to easily implement seven discrete extensions and a dedicated line out.

Planning out Extensions

First thing is to identify any jacks that want to share the same extension number. In my case I want the guest bedroom and the hallway to ring at the same time when a call comes in. I also want the guest bedroom and the electronics bench to share an extension so that the vintage computers that live in the guest room can be worked on at the bench without having to change any modem settings.

Next sort out the rest. Personally, I want all of the rest of the jacks to have their own extensions, so that any of them can directly call any of the other extensions. I also want the modular jack at the electronics bench to have two extensions. This allows me to have two vintage computers on the bench, and have them call each other. Because awesome. I also had to add in a branch that connects [AD] over to [SR] (the server room). This is a four pair cable, and I'm only using two of the pair for now.

Assign Pairs

First check to make sure you have enough pairs to cover all of the extensions you want. I actually ended up with a few extra.

Next, come up with [codes] for each of your pairs/extensions. You can see mine in blue on the pictured whiteboard. This makes it so much easier to keep track of.

Then, select which PBX extension phone number each of your pairs will end up plugged into (mine are in black). Again, much easier to keep track of if you write it down on the map.

Finally draw in connection lines for how each of your ext/pairs is connected through from the PBX to its modular jack(s).

Coffee Break

Triple check your work so far. Any errors you catch now will save you lots of effort later. I found that physically drawing out the network topology on a whiteboard was a great idea. These things are too complicated to keep in your head but obvious when drawn out.

Now that we have a plan, get your toolbox, it is time to rewire!

Step 4: Modifications

If you planned things out well, you probably do not need to make many changes at the modular jacks. In my case, I only needed to change two jacks.

Down in the Rabbit Hole, I changed the jack to be connected to Pair 2. To do this I disconnected Pair 1 (WhiteBlue/Blue) from the punch down block and punched WhiteOrange and Orange down into the block instead. Easy!

In the guest bedroom, I needed to change the hanging modular cord over to Pair 2, and wanted to clean up the hack job left over from who ever added the master bed room branch. This is the modification pictured above.

Looking inside, things are a bit of a mess, and only two of the four terminals are in use. I start by removing some of the shielding from Pair 2 (WhiteOrange/Orange). I loop those around the two unused screw terminals. I want the hanging modular cord to use this pair.

I loosen the other pair of terminals, and remove the hanging modular cord's wires from those. I twist both tip wires together (Green with Black) and both Ring wires together (Red with Yellow). Note that I did this wrong in the photo, remind me to get in there and fix that, haha (that's what I get for copying what they had done without thinking)! I then soldered them together and trimmed things back to a reasonable length (no more frayed sloppy wires). Now that I have a nice clean pair, I use the terminals to connect Tip (Green) to Pair 2 Tip (WhiteOrange) and Ring (Red) to Pair 2 Ring (Orange).

The branch that runs over to the master bedroom is connected to Pair 1, and that is the pair I want it on, so I leave that as is. Then I wrap the mess up in the second pair of branch wires to make things nice and neat. I screw the modular back into the frame, and re-affix the cover plate.

Now the master bedroom and the guest bedroom are on their own pairs, and everything is right and good in the world. That was easy!

With our modification's complete, it's time to patch the PBX into the system.

Step 5: PBX Connectors

A PBX, by its nature, has lots of things plugged into it. In this Instructable, most of those things are pairs of wires inside the wall. The PBX has a row of modular jacks, but these pairs of wires do not have modular plugs on them yet. How do we solve this problem for free? Reuse!

If you are anything like us, there is a box somewhere in the basement or garage filled with random extra cables and connectors from every bit of electronics you've ever owned. If you don't have a misc cables box, you should consider starting one. I went digging in our box and pulled out every single DSL filter I could find. These are the little pig tail boxes that come with DSL modems you place them between each phone and the wall jack (to keep the DSL modem and the phones from interfering with one another). The phone company always seems to send lots of extras. We don't use DSL anymore here, so these filters are a great source for parts. Each one has a short cable with a modular plug on the end. Perfect!

Making the PBX Connectors

I carefully remove the cables from a small pile of filters. Next, I strip the Green and Red wires on each of the cables. I tin them with some solder. These cables are too short for what I want to do, but that is alright I mostly want the connectors anyway.

I dig around the cables box some more and find a roll of single pair telephone wire. I cut this into equal lengths, one for each of my modular pigtails. I strip the ends, soldering and heat shrink tubing a pig tail to each one. At this point I have a nice pile of cables long enough to go from the mystery cover [AD] to my PBX. I break out my digital label printer, print out labels with the extension numbers, and stick one to each of the cables. You could do this with tape and a pen, but no matter how you do it ... label these! When it is time to plug all of these into the PBX, good labeling will make all the difference.

Tapping Phone Lines

At this point it is time to wire the PBX connectors up to the line pairs inside the wall. Refer to your map to see which EXT connector goes to which Pair in the wall. Twist Tip to Tip and Ring to Ring ... stripping, soldering, and heat-shrinking as needed. I used the PBX to test each connection as I made it.

Testing was the most fun part of the whole process. One person would sit at the PBX with a telephone connected to EXT 603. The other person would run to the jack in question with a basic telephone, then dial 603. If everything was connected, 603 would ring and then we'd get totally distracted by the novelty of chatting over our own private telephone system. "You hang up first" "No, YOU hang up first"

Once all of the lines tested good, I drilled a hole through the mystery cover, fed all of my pbx connectors through it, wrapped any extra pairs back around the bundle, and screwed the cover back in place.

Connect All the Things

Now that everything is wired up and tested, connect all of the PBX connectors to their appropriate jacks on the PBX. Congratulations, you now have a full blown telephone network running through your walls. Power up the PBX and try dialing various lines from other lines through out the system! If any part of your system isn't working the way you want it it, you now have the knowledge to get in there and make adjustments.

Step 6: Voice Over IP

One great thing about a PBX is that it can route calls in and out of the system through outside lines.

For our system, we connected an inexpensive Voice Over IP gateway to one of the outside line jacks on the PBX. This allows us to make and receive calls for free over the internet.

Very cool!

Step 7: Raspberry Pi, Etc.

Another interesting thing by a PBX (beyond your typical telephone / voice stuff) ... modems!

Here is a Raspberry Pi with a USB fax modem. It is connected to one of our extensions. You can do all sorts of great things with this. You could have it set up to send / receive faxes for example.

In our system, we actually have the Raspberry Pi set up as a Dial-Up ISP. When I'm playing with my vintage computers, I can dial up an internet connection. This capability opens up a whole world of fun projects, making these old computers useful and fun again.


I hope you enjoyed this Instructable! Please let us know what ideas you have for your private telephone system in the comments section.

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