Cheap and Easy Guide to Building a Private Telephone System





Introduction: Cheap and Easy Guide to Building a Private Telephone System

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.

For detailed information for taking what you learn in this Instructable and building it directly into the phone lines in your home check out another one of our Instructables Retrofit a PBX to Existing Phone Lines (opens in a new tab).


  • PBX
  • Old phones, modems, answering machines, etc.
  • Voice Over IP gateway
  • Modular telephone cables

Step 1: What Is a Private Branch Exchange (PBX)?

You may have run into these at work, at a hotel, or have seen references to them in television shows and film.

"Dial 9 to get an outside line"

"My extension number is 604"

"Let me transfer you to billing"

The magic box that makes all of this possible is a PBX. In large organizations, in order to limit the number of phone lines required (and to keep switching in-house), a private branch exchange system is installed. You can dial out to the larger phone network. People from the larger network can dial the number of the organization and then the extension they want to ring inside the private network. Or an incoming call might ring the secretary's phone, and they can in-turn transfer your call manually. Another great feature of these systems is the ability to dial other extensions on your network directly, never connecting to the outside network at all!

Until recently, this kind of technology was very expensive and difficult to set up. Ironically (now that most of us do not use our wired telephones) it is now embarrassingly easy and inexpensive to set and use systems that, not too long ago, would cost thousands and require staff to maintain.

So let's take a look at how, with less than $50USD, I was able to set things up so that I can now put telemarketers on hold (rickroll hold music?), transfer friends who annoy me over to my fax machine, make free internet calls, and ring the kitchen from my workshop to see how long I have until the cookies come out of the oven. Mmm cookies... (priorities).

Step 2: Collect All of Your Devices

Along with the 'Direct from China Ebay Special el-cheapo PBX', we need some devices to hook up to it. For my network, I collected the following:

  • Cheap Voice over IP gateway (for making and receiving calls over the internet)
  • A basic wall mount touch tone (for the shop)
  • A modern cordless phone with a built in answering machine
  • My restored antique telephones (so pretty!)
  • A Raspberry Pi 2 with an external Fax Modem
  • An acoustic coupler modem from the 1970s (because I'm just that cool)

Step 3: Connect the PBX to the Outside World

I really don't have any use for a traditional landline account but it is important to know that even if you do not have telephone service active on your landline it does still work for emergency calls. My particular PBX happens to have three outside lines. So I use one outside line to connect up to the old landline for 911. Though the only time I ever called, I got a busy signal, of course. Next time I'm going to try 0118 999 881 999 119 7253.

For daily use, what actually makes more sense these days is a voice over ip gateway. I have an inexpensive voip gateway connected to the PBX's principle outside line. This allows people to call the PBX from a traditional telephone number and allows me to make calls around the world at effectively no cost. These boxes have gotten very inexpensive and well integrated into all sorts of nice internet based phone services. Set up of the voip gateway generally involves syncing it with which ever online service you use (with the gateway plugged into power and to your local area network / wifi) and it 'just works'.

As you can see from the photos, getting the PBX to use the VoIP Gateway is as simple as connecting a short modular cord from the jack on the back of the gateway to the first outside line on the PBX.

You can complicate things by setting up what to do with incoming and outgoing calls in very great detail via your PBX's settings. However, these cheap PBX's come default from the factory to route any incoming call to ring all of the extensions, and to allow any extension to dial out on any outside line. I do not have any reason to change this behavior, so set up here was super silly simple.

Step 4: Connect All of Your Devices

Now connect modular cords between each of your devices to one of the EXT (extension lines).

The EXT lines act like a traditional telephone jack. They provide power, ring, voice, fax, whatever just like a real phone line would.

At this point you can provide power to your PBX and you have just set up your own private telephone system. If you pick up one phone and dial the ext number of one of the other phones, it should ring normally. Congratulations, you now have the coolest (and most over engineered) string and two tin cans intercom system ever!

Note: You absolutely do not need a voip gateway for any of the internal PBX functionality to work. You can dial extensions, transfer calls, party line, and put your significant other on hold all while blissfully disconnected from the outside world. On my PBX I can very easily make every single phone in the house ring from mine. This is perfect at dinner time when you want to let every one know.

Step 5: Applications!

So there is the basic set up. To wrap up, let's take a moment to talk about a few fun projects that having your own PBX enables.

A red 'hotline' phone. If you work from home and have kids, this is a great way to let them know that they can always reach you while you are in your office but with just enough of a 'it has to be important' barrier to keep the distraction level down.

A Raspberry Pi virtual fax machine. Even in this internet based world, you still run into situations where you need a fax machine. It is surprisingly easy to set up a Raspberry Pi to send or receive faxes.

A real telephone ringer. Find an old telephone 'subset' box to connect to your PBX. Early telephones did not have a ringer inside the phone. They relied on external subset boxes with big brass bells. When a call comes in, hear that jangling ring from the good old days again.

A dial up internet service provider. Here is another one that a Raspberry Pi works great for. With a Raspberry Pi and an external usb modem connected up to one of your PBX EXT lines, you can 'dial up' from any of the other EXT lines on your network to your own ISP. It's a quick "sudo apt-get install pppd", editing a few lines of config text, and you are there. If you have your PBX set up with VoIP, you could even dial into your ISP from anywhere.

A Bulletin Board Service (BBS). Have any vintage computers in the attic? A PBX can be the simplest way to get them on the internet. Old modems work great over a PBX. An Atari's 300 baud acoustic coupler, a Commodore 64's 1200 baud modem, or even the 14.4k modem in a 486 dos box ... any old machine that you would have run your BBS on back in the day can be easily set up so that people can dial into it over the internet.

Let us know what ideas you have!

I hope you find this Instructable helpful or at least interesting. Don't let old phones collect dust in the garage, hook them up and have some fun!



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Please be positive and constructive.



Do you know what kind of device I could use to enable a tannoy type speaker arrangement on one of the extensions. The objective is to dial that extension from a different extension and it would automatically answer so I could just relay a message to be 'broadcast' from that device (a speaker basically but with the capacity to automatically answer whatever 'calling' signals the PABX sends to it when I dial it).


So, if I have 3+ phones and a PBX, can I dial between the 3+ phones without the PBX being connected to the outside world? My idea is setting up an intercom system between the floors (basement, 1st, 2nd) and between two parts of the basement.

Okay... So can I just grab some phones, connect them to the pbx and make calls? And am I required to have anything plugged to Line 1,2,3?

Also can 2 calls run at once?

if you only have one set of phone lines from the phone company then you can only make one call. You could make one outside call and one internal call calling from one phone to another inside your office.

im new in the instructables and i want to build your project, let say i have all the equipment needed,i notice that you use raspberry pie and i have zero knowledge on setting it up or programming it. can you help me with that?

Hi is there a cheap Telephone PBX that will also allow ISDN Phones to work and allow puslse and tone dialing and will ring old rotary phone?

Thanks for Anwsering and Best Regards

Sorry, not sure on that...

I wonder if it would be possible to hook this system up to some python code. I have always wanted a phone with blacklisting/whitelisting features. Let's say you wanted a whitelisted phone. Only numbers that are on a pre-approved list (in a formatted text file) would cause the phone to ring. An incoming number that is not on the pre-approved list could go to a pre-recorded message and then disconnect.

That is a nice idea too bad it looks like we won't get the answer