Hack a VOIP Box Into a Telephone Intercom




About: I don't use instructables any more, so please consider all my projects here as archival.

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:

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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.

Step 2: Logging On

First thing you have to do connect your PAP2T to your home network, open a web browser to its IP address and log on as a user. Then choose the link to log on as an administrator.

Step 3: Change to Advanced View

Change the view to "Advanced". Otherwise you will not see the settings that have to be changed.

Step 4: Switch to Regional Settings Tab

Switch to the "Regional Settings" tab

Step 5: Optimize Ring Settings

To most reliably and loudly ring a standard metal gong ringer phone, make sure your settings are as follows

Step 6: Gain and Impedance

These are optional, but you may need to adjust them for best quality. Normally set your gains to "0", but if you find it too loud, turn it down to "-3". Set impedance to "600".

Step 7: How It Works

Before we go further, let's talk about how this works. Normally, your VOIP box goes out over the internet and authenticates and communicates with a VOIP server. VOIP traffic normally uses TCP port 5060, with additional lines at 5061, 5062 etc... We're not going to use a VOIP server, though, so we don't need to authenticate to anything. Basically, we're going to have the adapter talk to itself on what is called the "loopback" address ( We are going to configure each line with a port and a name. We are also going to enter a dialplan for each line that causes it to call the other line as soon as the phone goes off-hook.

Having the PAP2T plugged into a hub is only to keep the box thinking the network is available. It doesn't actually need to talk to anything else on the network.

Step 8: Select Line 1

We will begin with configuring line 1

Step 9: Enable Line 1

Make sure line 1 is enabled

Step 10: Configure Line 1 SIP Port

Configure the line 1 SIP port as shown

Step 11: Configure Line 1 Proxy and Subscriber Info

Configure the Line 1 proxy and subscriber info as shown. We're basically telling it to make and answer calls without talking to a proxy and to not bother authenticating.

Step 12: Configure Line 1 Codec

Configure line 1 to use the G711u codec only. Other codecs will not work.

Step 13: Configure Line 1 Dial Plan

Configure the dial plan for line 1. This is what causes it to call line2 when line1 is taken off hook.

Here it is so you can cut and paste:

Step 14: Enable Line 2 and Configure the SIP Port

Enable line2 and configure the SIP port.

Step 15: Line 2 Proxy and Subscriber Settings

Here are the line2 proxy and subscriber settings

Step 16: Configure Line 2 Codec and Dial Plan

Configure the line2 codec and dial plan as shown.

Here is the dial plan so you can cut and paste:

Step 17: Plug in and Test

We're done configuring, so be sure to click "Save Settings" in the PAP2T web interface. You only have to do it at the end, it is not necessary to do it between tabs. Let the box do its reboot. Now, plug in your phones and try it.

Here's a shot of the back of the PAP2T and the switch. Plug a phone into each line and the ethernet jack into your switch or hub. If you're connecting to the existing house wiring, be sure the line has no connection to any other VOIP adapter or the phone company.

Step 18: Configure Ring

If it worked, you might want to configure your ringdown for a ring different from your regular telephones. Here is where you set it. Basically, switch another ring cadence with Ring1 Cadence. Be sure to click when done.

That's all there is to it! Enjoy your new telephone intercom/ringdown. Even if you bought your PAP2T new, you likely saved at least 66% off the cost of a basic ringdown module.

Hack It! Challenge

Participated in the
Hack It! Challenge

1 Person Made This Project!


  • Instrument Contest

    Instrument Contest
  • Make it Glow Contest

    Make it Glow Contest
  • STEM Contest

    STEM Contest

32 Discussions


Reply 2 months ago

Before entering config for this project, you have to reset this device.
To reset the PAP2T to factory defaults follow these steps:
  1. Plug in power adapter.
  2. Plug in a phone to 'Phone 1′ port of the PAP2T.
  3. Dial ****
  4. You should hear 'Configuration Option Menu'
  5. Dial 73738# (RESET#)
  6. Press 1 to confirm the reset.


7 months ago

I did it with VoIP Linksys WRTP54G !

73's SQ6RDP

1 reply

Reply 6 months ago

how were you able to do this, mine is locked.


2 years ago

Hey uhclem, i was wondering if you know how can i setup a call forwarding if no answer.

Example, i dial de second extension, but no one answers. how can i forward this call an asterisk box for example?

I have found this info telling how to configure, https://sbkb.cisco.com/CiscoSB/GetArticle.aspx?doc...

I did not implemented yet, but if i understood correctly, i will configure a call forwarding number that hits a secondary dial plan and sends the call to my asterisk box.
Do you think that this can be done?

Thanks again!


7 years ago on Introduction

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:

2 replies

Reply 2 years ago

Hy Gurr, just found this instructable, and you said that it worked with a loopback plug. Did you do it yourself ? if so could you tell me how ? I've seen to kinds of loopbacks, and they are wired differently, I don't know which one to use.... one is here : https://www.youtube.com/watch?v=K1cjtHUDldY the other here https://www.youtube.com/watch?v=9SdKQLV4vC4 many thanks


Reply 7 years ago on Introduction

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


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:


1 reply

Reply 3 years ago

Both you guys are the best!

I have a cenario where the internet is not available all the time, so i need the pap2t to be useful in both cenarios, as a intercom module and also as a voip line!

Thank you so much!


3 years ago

Thanks for this amazing article. I had my intercom work in less than 30
mins. Well explained. I had a quick question. I am using my internal
cat 5 wiring at my home and I only have one line available for this
intercom. I would like to connect my telephone on the 3rd floor to the
telephone in my basement using this setup. How can I do it.


5 years ago on Introduction

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.


5 years ago on Introduction

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,

2 replies

Reply 5 years ago on Introduction

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.


Reply 5 years ago on Introduction

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.


6 years ago on Introduction

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,


6 years ago on Introduction

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

2 replies

Reply 6 years ago on Introduction

You should be able to do it like so:
(line1 ata1 dialplan)
<#1:> S0 <:line1@:5060>
(line1 ata2 dialplan)
<#1:> S0 <:line1@:5060>


Reply 6 years ago on Introduction

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


6 years ago on Introduction

I am a VOIP subscriber. Can I set something up like this with a neighbor and use it via our ISP?