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

Picture of 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)?

Picture of 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

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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

Picture of 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

Picture of 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!

Picture of 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!


ДимоБ (author)2017-04-06

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?

ДимоБ (author)ДимоБ2017-04-06

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.

rtionghoy1 (author)2016-08-05

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?

Mitjak4 (author)2016-05-23

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

Tymkrs (author)Mitjak42016-05-23

Sorry, not sure on that...

Dhayes100 (author)2015-08-28

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.

PhillipP2 (author)Dhayes1002016-02-04

If you load freepbx onto a raspberry pi it will do just what you are asking. Here are the instructions for that.

MatthewL17 (author)Dhayes1002015-09-14

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

TechPaul (author)Dhayes1002015-09-01

there is no caller id built into these systems, they rely on DTMF signalling to active features. the call routing is based on the number dialled from the PBX handsets.

jtetreault2 (author)Dhayes1002015-09-01

Just get/use the open source Asterisk based pbx system.... i like the freepbx

Tymkrs (author)Dhayes1002015-08-29

If your landline/voip supports caller id and your pbx passes caller id through (our cheap one does), then your raspberry pi / fax modem can get the caller id data (with python, etc.) and transfer the incoming call to an extension that actually rings a phone. Though, if you are using voip, there may be white listing features built into your service (there is on ours).

rickkeil (author)2015-11-27

It should be noted that this idea is awesome but this particular PBX system cannot interpret the dialing of a rotary phone. That means that you cannot simply plug in a rotary phone and dial other phones on the internal network, nor can you dial the outside world with a rotary phone and this system. The Panasonic 308 PBX system can interpret a rotary phone, so if you want to use this with old rotary phones search on ebay for a Panasonic easa-phone 308 system.

bblack6 (author)2015-09-14

I'm new to all this but it looked like fun. (It was.) Got the same PABX box. The directions take a while to digest. I assume C.O. means Call Out? It never says... And I still don't know what Flash Time Settings is for. Had a problem with only getting a dial tone from a receiver every other time but that went away when I eliminated the calling limits. Was hoping I could find a way to have the option to call in ring all extensions, but I can live with it. Ironic that I'm going to have to find a land line to use to record a decent sounding message. I don't have one anymore!

TechPaul (author)bblack62015-10-28

CO means Central Office which is the telephone exchange most of the time.

Tymkrs (author)bblack62015-09-14

I believe that CO is technically 'central office', but yeah they are the trunk connections that get you out of your local pool into something else.

Flash Time is how long of a 'flash' or 'pad' the pbx expects from the ext phones when they want to trigger things like adding another ext to a party call etc. A flash is when an exention hangs up for a very brief moment but comes right back off hook

In ours, you can set up all kinds of behavior as to what extensions to ring when a call comes in on a CO line. If you did indeed track down the same model then it's in there (hidden among the commands listed in the mysterious and baffling manual hahaha).

bsims1 (author)2015-09-07

I think most houses will be wired with bridge taps. So you'll really only have a possibility of three separate extensions, one for each pair. In my opinion, the best use for old phone wiring is to use it to pull three new CAT6 cables without digging around in the walls with a fish tape.

Now, if you really want to geek out with phone systems, build your own exchange like this guy...
Then make an instructable for it so I can read it.

TechPaul (author)bsims12015-09-08

Or if your house has a nice Cat5\6 patch panel(doesn't yours) it can be easy. The problem with the linked PBX design is it very expensive, especially when compared to a finished product like these units. The optoisolators would cost nearly as much as this unit, if I visited my nearest electronics shop. Just buy one of these , they are not perfect but with everything going VOIP the day of the PSTN may be over soon. Although my house is to be upgraded soon from ADSL2 to vectored VDSL so the old phone might still get some use. We were supposed to get Fibre to the house but now it looks like may take another 10years , probably just move to an area with Fibre.

bsims1 (author)TechPaul2015-09-08

Yeah, I hear ya on the cost. I would never go through all the trouble of building one, but I like to have it in my zombie apocalypse manual next to the section on building a BBS.

I did structured wiring in my old house. I just mounted a switch, 110 block, and an F-connector distribution block to a backboard. Later I got a cool swing out rack and panels with BNC and CAT6 keystone inserts, but decided to move before I could install it all. My new place doesn't have room for a rack, and isn't worth it. I'll probably build another backboard under the stairs when I run new cable. One day I'll have a use for my cool rack, if I don't sell it first.

I was looking at Asterisk, but the ADA boxes for connecting a bunch on analog sets are too expensive to bother with that.

TechPaul (author)bsims12015-09-09

last house had multiple RJs going back to 3 x 24port patch panel, this house has 3 Cat5s . the big difference , last house a wifi card cost $600 ea when I put the cable in.


Astinsan (author)2015-09-08

Modems don't work very well on voip. you need to make sure you have v.34 voip service. it is uncompressed so you will get 9600 speed...

Astinsan (author)2015-09-08

I run cable for a living and put network devices in.. Fish tape isn't as hard as it looks. You just need a cool day to do it.

Kamoku_Katai (author)2015-09-08


Tymkrs (author)Kamoku_Katai2015-09-08

Have you tried turning it off and on again? :D

spoo76 (author)2015-09-07

awesome instructable. love the I.T. crowd reference lol

Tymkrs (author)spoo762015-09-08

Haha thanks! :) I can still see the fire around Moss!

AdrianW12 (author)2015-09-07

Old Dreamcast modem should work on this... If only there was somewhere to connect to ;)

jcrichard (author)2015-09-07

That HAK5 sticker!

Tymkrs (author)jcrichard2015-09-07

Trust your technolust!

mattcintosh (author)2015-09-01

Can I run some cool SIP phones over PoE? The Cisco phones are cheap, but the Polycom still are a bit pricy.

mattcintosh (author)mattcintosh2015-09-01

I have an Avaya 9641G with a color screen, but it doesn't get past "waiting for DHCP"

richms (author)mattcintosh2015-09-07

It probably is set up for the voice to go on its own vlan, so wont get an IP when plugged into a network without that vlan. See if there is some button combo to factory reset the phone.

chrwei (author)mattcintosh2015-09-01

for a sip phone, you need a sip phone server, such as Asterisks. you may also need to set up a tftp server for the phone to get its config from, which may also require having control of the DHCP settings, most home routers won't allow it. Asterisks guides should cover all that.

TechPaul (author)mattcintosh2015-09-01

this is for old school phones

dhergott (author)2015-09-01

Where can low cost PBX's be purchased like the one your demonstrating?

danyboy666 (author)dhergott2015-09-07


Tymkrs (author)dhergott2015-09-01

Our unit happens to be a SOHO-PBX SP-208M (sourced via ebay, imported from china)

TechPaul (author)2015-09-05

Checked and the output voltage is 49V, close enough to the normal exchange voltage, this means it will be able to go a couple of Km with no problem. Just check your output voltage as I bought my box in '97 so it may have changed in the meantime, but the Chinese don't like to make too many changes unless it saves a lot of money.
One thing, I did go thru the manual and made a cheat sheet as the manual(back then) was badly organised. And I started work on a Visual Basic program to program the PBX via a dialup modem. It was easier just to use the cheat sheet than complete the program. Maybe an arduino with a DTMF encoder would do, the actual code isn't too hard, just the number of boxes the info needs to go into seems to get very large.

Tymkrs (author)TechPaul2015-09-05

And here's our other instructable to making a DTMF encoder!

TechPaul (author)Tymkrs2015-09-05

I saw that, the problem with PBXs are the routing tables, ie if you dial an area code better served by VOIP or emergency which should not go thru VOIP or is it a local number you get for free on your plan. Throw in a GSM dialler and things start getting tricky. A GSM/3G dialler if used properly on these and in the RIGHT circumstances can save money. Then there is the access barring, stopping some one just plugging a phone into the socket/line 600yards from the house and dialling Tanzania. I'm sure with this PBX you can simply not assign an outside line to the remote gate connection.

CJM3 (author)2015-09-01

What's the longest distance from the PBX you can have one of your devices?

CJM3 (author)CJM32015-09-03

I was just thinking of putting a line at the gate 800 ft. away for the delivery people to call in. I don't like giving my cell number or posting it at the road for anybody to see, and the gate is always closed/locked. So I think I'm going to try this. The cost seems reasonable enough that I can test it and if it doesn't work the way I want I don't have to give the "little lady" much of an explanation.

Tymkrs (author)CJM32015-09-01

The farthest phone set we have away is about 2 levels up? I'm sure it could do more though - since really, you're mostly dealing with phone lines and phone lines go for longer distances than that :)

srilyk (author)Tymkrs2015-09-02

As an interesting and relevant aside, old telephone systems used to use barbed wire to connect farms in a party line. They already went to the trouble of stringing wire for miles and miles, it was way cheaper to just put some clips on the wire on this and that end.

Of course you do need at least *2* wires, so chain link fence is probably right out.

Tymkrs (author)srilyk2015-09-02

Hah, that's awesome :)

TechPaul (author)CJM32015-09-01

it depends on the output voltage of the PBX, the higher the voltage the longer nthe distance. As a rule modern phones need about 30mA to operate properly so if you do have a problem check the amount of current , with the off "off-hook" ie handset up.

CJM3 (author)TechPaul2015-09-01

Thanks I was hoping maybe their was a spec sheet but nothing on the model listed from the manufacture I could fine yet, but I do have a meter so I can check it if I buy it.

TechPaul (author)CJM32015-09-01

I have one nearby, I might be able to find out but it is probably 24V which is a typical PBX voltage as long distances are usually not require. Even a 24V system should get at 1Km range I have seen people try for too long a distance and get into trouble , then a quick check of the loop current tells the tale, I'll leave ohms law up to you.

TechPaul (author)TechPaul2015-09-01

and the phone will probably need at least 6-8V at 30mA to operate, maybe more they vary quite a bit

GrfxGawd (author)2015-09-03

Everything you need to know about PBX, VOIP, IP PBX, and the geeks that drool over such topics, AND the software to make it all work.
Resources for any level of scaling you want to consider. Many are BPX and VOIP professionals. More than a few just want control over their own phone, pay close to nothing (or less) and get more features than they could actually pay someone to provide. For the tech inclined, it can be in place and effectively hands off in under two hours.

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