Hack a VOIP Box Into a Telephone Intercom

Ever want to use telephones as an intercom? This instructable shows you how to reconfigure a VOIP adatper to behave as a "ringdown" - meaning, when one receiver is picked up, the other rings until it is answered and vice-versa.

It's easy to find standard telephones - they are cheap and plentiful at flea markets, second-hand stores and yard sales. Telephones made prior to the AT&T breakup (1984) are also very well made - they were rented to the customer and were built to last forever. Some people (like me) even collect them!

Telephones, however, have specific needs for power supply and ringing which make them somewhat difficult for the experimenter to use. Telephones require about 20ma of talk current at 48v DC for the carbon mike and dynamic speaker to operate and they also require around 90v RMS AC at around 20 Hz to ring properly. Telephones also expect a line impedance of about 600 Ohms.

Commercial solutions exist for creating a ringdown, but they are expensive. Mass consumer technology to the rescue! Most any 2-line VOIP box has exactly what we need to power the phones and cause them to ring - for a lot less money.

See the video:

Step 1 Items Required

This instructable requires the following:

- an unlocked 2-line Linksys PAP2T VOIP adapter. Other brands will likely work, so long as the have two lines and you can figure out the settings if the names are different. It is important, though, that your adapter be unlocked - meaning you have access to all the settings. Most VOIP providers give you a locked-down box, either through custom firmware or a password. It may be possible to flash such devices to unlock them, but that is beyond the scope of the instructable. A new, unlocked PAP2T is inexpensive.

- a pair of land-line telephones - any kind will work, even rotary, since no dialing is required.

- an ethernet hub or switch. The PAP2T will not work if it detects the ethernet interface is unplugged. It might be possible to overcome this with an ethernet loopback plug - I have not tried it.

Be careful attaching the VOIP unit to existing phone lines. You want to make sure any line you plug in is not in any way connected to another VOIP box or the phone company. Use a voltmeter to see if extra lines in your home are active or not and be careful, the ring voltage hurts!

If you run wires outside your house, consider using a lightning protector at each point where the wire enters a structure.

Wireless phone extension adapters are available but see if you can borrow one before you buy, as they don't always have the best range or clarity. I have a set purchased cheaply secondhand, and while they worked from one side of the room to the other, they would not work between floors.

Comments

[gurr]

It works with a loopback konnection/plug instead of a network.

You may of course make experiments with dial tones, and ring cadences etc.
This gives a good old British sound. Changing the sounds a little, and it could be old style sounds from USA or Norway:

[uhclem]

That's great news. I figured it would but did not have one to test. Much better than having a hub around.

[richms]

Awesome idea.

Do you know if its possible to make a basic menu in the dialplan so I could have 2 of these connected to eachother but no internet and be able to dial the others with a single digit press?

[uhclem]

That should work. You'd have to assign a static IP to each PAP2T and reference it in the dialplan. I only have one PAP2T so I can't test it myself.

[uhclem]

In this example, each line has to dial #1 to reach the other:
(line1 dialplan)
<#1:> S0 <:line2@127.0.0.1:5061>
(line2 dialplan)
<#1:> S0 <:line1@127.0.0.1:5060>

If you set up G711u as a secondary preferred codec you can have this as part of a real dialplan where the VOIP box actually gets calls. I do this at my home. Each line can make and receive calls AND call each other internally.

Example:
(<:1508>[2-9]xxxxxxS0|*xx|0|00|[3469]11|1[1-9][0-9][0-9]xxxxxxx|1900xxxxxxx!|<#1:> S0 <:line2@127.0.0.1:5061>)

You can set this up with a pair of PAP2Ts and have it happen over the internet.

[twocvbloke]

This is a pretty cool setup, and pretty easy to do, and I too just happen to collect old phones, so this could be handy to test the function of new acquisitions aswell as be a useful intercom, I have to try this at some point... :D

And using the Cadence options, you could have a US-style 2on-4off ring for US telephones, and for UK phones, a BT-style ring (0.4on,  0.2off, 0.4on, 2off), cos a US phone on a UK phone line just doesn't sound right (sounds nice, but not right!!)... :P

[uhclem]

Glad you like it. It is so much easier than building your own ringdown and much cheaper than buying a 'real' one.