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).

Cheap and Easy Guide to Building a Private Telephone System

Step 1: What is a private branch exchange (PBX)?

ou 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)Cheap and Easy Guide to Building a Private Telephone System schematic

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.

 

For more detail: Cheap and Easy Guide to Building a Private Telephone System




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