Hack a VOIP Box Into a Telephone Intercom

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Introduction: 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:

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 (127.0.0.1). 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:
(S0<:line2@127.0.0.1:5061>)

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:
(S0<:line1@127.0.0.1:5060>)

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.

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37 Comments

0
jasongordon1
jasongordon1

10 months ago

I cant get my phones to ring :-(
Linksys SPA-2102
5.2.3
Any ideas?

Screen Shot 2020-08-16 at 6.42.08 PM.pngScreen Shot 2020-08-16 at 6.42.19 PM.pngScreen Shot 2020-08-16 at 6.42.28 PM.png
2
jamesianjordan
jamesianjordan

12 months ago

It's taken me ages to get round to posting this but I made this last year as a phone line between my house and the kids' playhouse at the end of the garden. I use a robust old payphone in the playhouse and the phone line is laid properly through a trench in the ground to the house.

A year later it's still working perfectly and is a big hit with visitors. Children who visit are obsessed with it and adults can't believe there are working phone lines any more. Thanks uhclem for a fantastic instructable.

Having tinkered around with it a fair bit to get the best, most reliable setup I thought I should share my findings.

1. I use a Cisco SPA112 2-Port Phone Adapter.
I don't think the PAP2T is made any more. As far as I can tell the ones on Ebay are all fake. I've had two, both were clearly fake, were unreliable and then broke after a few months. The SP112 is genuine, doesn't cost much more and has an almost identical config menu so the settings in the guide still work and are easy to find. It's rock solid - I've left it on for days with no problems. That was impossible with the knock-off PAP2Ts.

2. It works without an internet connection by using a network loopback cable provided you set a static IP address first.
I needed a way to use it without an internet connection but the PAP2T and SP112 don't supply power to the phone lines until they get an IP address.
I followed a YouTube guide to make a network loopback cable. That didn't work until I changed the SP112 network settings to have a static IP address. That works and it's been happily phoning away for a year without going near the internet.

0
gurr
gurr

9 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:

setup.jpg
0
brighterfusion
brighterfusion

Reply 1 year ago

No, I tried using a loopback plug and it didn't work.

It did, however work on a router which wasn't connected to the Internet. It's just looking to be assigned a LAN address. An ethernet switch won't work either.

0
Simoncu
Simoncu

Reply 4 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

0
uhclem
uhclem

Reply 9 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.

0
Simoncu
Simoncu

Question 1 year ago

Hello I don't know if someone will look at this, but I tried it, but ! I have no phone line (well I have a phone line but only internal of my house) so should I put a transformer on the wires to get the whatever voltage is needed on the phone wires ?

Thanks

Simon

0
willys48
willys48

Answer 1 year ago

Hey, I just got one off ebay, no you will not have to power up the line. The adapter does it all. Instructions are very easy to do. The 2 paste in lines are different line 1,2. So just copy one at a time. you can save the changes when you are all done. I saved at intervals and modem reset couple times. But it works! I hooked it into back of a router that is way away from the modem. Also looking to see if a loopback adapter will work so it does not even need to be connected to use. It does have to be connected to go in and change. Make sure an unlocked unit is what you get. It looks like you have one already but anyone doing this I thought I would share my experience. Very cool and saved me money on a 100 plus dollar unit.

0
box99
box99

6 years ago on Introduction

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

0
vieuxpalais
vieuxpalais

Reply 1 year 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.

0
sq6rdp
sq6rdp

2 years ago

I did it with VoIP Linksys WRTP54G !

IT WORKS!
73's SQ6RDP

WP_20190410_23_19_40_Pro.jpg
0
bopeep32123
bopeep32123

Reply 2 years ago

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

0
lpucci
lpucci

3 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!

1
ghettocottage

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:

https://www.instructables.com/id/VOIP-Phone-AND-intercom-system

0
lpucci
lpucci

Reply 4 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!

0
NoelD28
NoelD28

4 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.

0
hansonsux
hansonsux

7 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.

0
magicstop
magicstop

7 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,

0
uhclem
uhclem

Reply 7 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.

0
magicstop
magicstop

Reply 7 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.