Picture of 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:
box9911 months ago

Any reason a PAP2 (vs a PAP2T) wouldn't work for this application?

Several people (myself included) were curious about linking additional boxes/phone lines to a system like this. I was able to set this up so I wrote another instructable:


hansonsux1 year ago
Corrections: 1) It is 48vdc open circuit. Lift the handset offhook, and it drops to about 6 volts when the line is loaded to about 20ma. To try and put 20ma at 48vdc will let the magic smoke out. 2) The ringers are designed so that a minimum of about 40vac will ring them. 90 volts is a nominal value sent from the co so that something above 40v will be received on the other end of miles of cable.
magicstop made it!1 year ago

This works so well when I am testing component changes and ring sensitivity. I found some manuals at CISCO for the PAPT2 that might be helpful to some.

PAPT2 - Admin Guide:


PAPT2 - Provisional Manual:


End User Guide:


magicstop1 year ago

First and foremost: Thank you so much for producing this Instructable as now I can test my Telephony device builds far away from MaBell Phone Lines.

I only need one phone and my device that puts a load on the line 2 and line 1 rings until I pick it up. I then am able to listen to the recording my device sends to the line 1 phone.

This PAPT2 configuration will now allow me to test my ring detection circuit and inline live service testing of voltages that make it pass my audio transformer.

I do have a question though, Using VOIP like this I would assume that Ring voltage should not fluctuate as it does with regular phone lines do to storms and area usage of neighborhood phone lines ? Is that correct ?

As a note to others. I left the Ring Voltage at 85 volts as it mis-ringed at 90 volts.

Again Thanks,

uhclem (author)  magicstop1 year ago
I'm not sure what the standards are for phone lines as far as maximum voltages. I think the lighting arrestor installed by the phone company works at 300v. I would think a pair of zener diodes would work to clip each side of the signal down to something safe on the equipment side of the audio transformer.
Yes, that's my understanding as well. Many circuits I have seen use zener diodes on the audio equipment side of the audio transformers but, I have also see them on modems prior to the audio transformer as well.

Thanks for reply and again for doing this excellent Instructable.

joepaton1 year ago
Wow, this is great!
As a telephone collector myself, I have made basic intercoms in the past.
This is so much better because it rings the bells! other intercoms for old phones require a separate electronic buzzer.

Following your instructables, this took me about 30 mins to get it up and running.
I have started playing around with the ring to make it sound more like my Australian ring but I will need to correct it a bit.

Thank you so much for sharing your knowledge!!!

Kind Regards,
kbradt2 years ago
i was wondering is there a way to get 2 linksys voip adapters to call one another the one the voip adapter that i have is a UNLOCKED Linksys SIPURA SPA1001 VoIP Phone Adapter 1 FXS that only has 1 rj11 port and the other voip adapter that i have is a
Brand New Unlocked Linksys Cisco PAP2T Internet Phone Adapter with 2 VoIP Ports so can you plese make a totoral that will help me with the project that i want to do and i also want to make a adapter that will automaticley ancer the pnone and send the audio to a pa amps 600 ohm telephone audio input connecter
uhclem (author)  kbradt2 years ago
You should be able to do it like so:
(line1 ata1 dialplan)
<#1:> S0 <:line1@:5060>
(line1 ata2 dialplan)
<#1:> S0 <:line1@:5060>
kbradt uhclem1 year ago
well i need the dial plain to have the reciveing voip adapters ip address in it and i need help with it so can you help me plese
posloq2 years ago
I am a VOIP subscriber. Can I set something up like this with a neighbor and use it via our ISP?
kbradt2 years ago
i need help with a poject im trying to get 2 linksys voip adapters to call eatchor and i dont know what configerashions i need and i dont know what the dial plain is for them to work so can some one plese help me with the project
kbradt2 years ago
hi i need help i need to reset my linksys sipuraspa voip adapter to the factory defalt
Pirus3 years ago
You Rock!

I took an old Vonage pap2t, plugged in a phone and did the ****110# to get the IP. Then did a factory reset with ****73738# thankfully I didn't need to enter a password just 1#

Followed your directions!

Two notes for you though...
I had to set the voltage to 86 (default). Setting it to 90 caused the ata to reboot. Also I had to leave the ata plugged into my network. A loop back plug doesn't work since it doesn't activate the phones until it has an IP.
AxvoiceInc3 years ago
You get all these configuration details when you buy a new voip account. Don't worry if those instructions are a little bit different. Although most of the times, they are approximately the same as mentioned above.
dv38943 years ago
Could you create a network of phones using multiple VOIP adapters connected through the switch or hub?
uhclem (author)  dv38943 years ago
You can give each VOIP adapter a static IP address and then refer to it via that address in each units dialplan.

<#1:> S0 <:line2@>

You can also modify the dialplan so that dialing a certain number on the phone calls a certain VOIP box.

There's a limit to the size of the dialplan, so you can't become the phone company this way.
bponaugle3 years ago
I followed the directions and it worked like a champ. Thanks! This is a great idea.
I realized that I could make this process less painful using the linksys spc compiler. I generated a template and made the changes in the template, compiled them into a cfg file and put it on a server.

To load the file on a unlocked pap2x with internet access follow these steps:

Get access to your PAP2x web interface

1) Plug adapter in a network with DHCP.
2) Dial ****110#
3) Listen for IP Address
4) Enter IP Address into your web browser (For example.
5) When you see the Linksys PAP2x Web Interface select the admin login.
once authenticated enter http://(IP address from 2ed step)/admin/resync?http://dogtransport.org/profiles/ringdown.cfg
6) The adapter will reset and work as described in the instructable
gurr3 years ago
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 (author)  gurr3 years ago
That's great news. I figured it would but did not have one to test. Much better than having a hub around.
richms3 years ago
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 (author)  richms3 years ago
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 (author)  richms3 years ago
In this example, each line has to dial #1 to reach the other:
(line1 dialplan)
<#1:> S0 <:line2@>
(line2 dialplan)
<#1:> S0 <:line1@>

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.

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

You can set this up with a pair of PAP2Ts and have it happen over the internet.
twocvbloke3 years ago
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 (author)  twocvbloke3 years ago
Glad you like it. It is so much easier than building your own ringdown and much cheaper than buying a 'real' one.