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Anyone know of an adapter for this phone jack?


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#1 flarn2006

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Posted 15 January 2012 - 08:15 PM

I'm trying to figure out what number my alarm system dials. Problem is, when I run the comm test and pick up the phone, it seizes the line (presumably so a burglar can't pick up the phone and prevent it from communicating.) I figured I'd open the alarm panel and connect a splitter to that same jack, or use an extension cable to connect it to a regular jack, but the plug is wider than a normal one so I can't plug it into any other jack. I know it's a phone jack, since I've plugged a phone into that jack before and successfully called an ANAC, but the plug that's normally in there can't be plugged into a normal phone jack. Any ideas on what I can try? Any ideas on measures I can take to prevent it from seizing the line so I can listen to the DTMF tones? Or anything like *69 for outgoing calls?

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Edited by flarn2006, 15 January 2012 - 08:17 PM.


#2 JmanA9

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Posted 16 January 2012 - 01:11 AM

If you don't have any other phones connected to the line the alarm uses, you'll need a lineman's headset. When you have the thing in "Monitor" mode, you can passively listen to the line without the alarm knowing. You could pop the cap off the jack and clip onto the enclosed terminals. If you're good with electronics, you could build one.

If that's not an option, you're not out of luck. Assuming you have other phones connected to the line used by your alarm, you could also monitor the phone line through one of these phones. This works best with telephones that do not have an AC power source: old Western Electric or Automatic Electric phones are perfect. If you connect a suction-cup recording patch to the base of the phone you will be able to hear the phone line without picking up the phone.

Also, if you have a VOIP adapter (like a PAP2), you could connect it to the alarm, let it dial out, and view the last dialed number in the web interface.

#3 CaffeinePT

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Posted 16 January 2012 - 01:31 AM

I'm trying to figure out what number my alarm system dials. Problem is, when I run the comm test and pick up the phone, it seizes the line (presumably so a burglar can't pick up the phone and prevent it from communicating.) I figured I'd open the alarm panel and connect a splitter to that same jack, or use an extension cable to connect it to a regular jack, but the plug is wider than a normal one so I can't plug it into any other jack. I know it's a phone jack, since I've plugged a phone into that jack before and successfully called an ANAC, but the plug that's normally in there can't be plugged into a normal phone jack. Any ideas on what I can try? Any ideas on measures I can take to prevent it from seizing the line so I can listen to the DTMF tones? Or anything like *69 for outgoing calls?

Posted Image


According to Wikipedia it is either:

RJ31X 8P8C Allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed ahead of all other equipment. (Only 4C are actually used)
RJ38X 8P8C Similar to RJ31X, with continuity circuit. If the plug is disconnected from the jack shorting bars allows the phone circuit to continue to the site phones. (Only 4C are actually used)

More information about them from Stratmando:

* RJ31X: 8P8C (although usually only 4C are used), Often incorrectly stated as allowing alarm (fire and intrusion) equipment to seize a phone line, the jack is actually used to disconnect the equipment from the phone line while allowing the phone circuit to continue to the site phones.

* RJ38X: 8P8C, similar to RJ31X, with continuity circuit (this is the one that should be used for fire and security applications because of the continuity pair. If the plug is removed from the jack a short is created between pins 1 and 8 therefore if correctly wired will trip a panic or fire alarm.)

The way to tell them apart (not really necessary because an adapter for one should work for the other):

Minor difference between the RJ38X and RJ31X: There is a strap between pins 2 and 7 on the RJ38X. (Pansophic in this thread.)

#4 flarn2006

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Posted 16 January 2012 - 09:43 AM

If you don't have any other phones connected to the line the alarm uses, you'll need a lineman's headset. When you have the thing in "Monitor" mode, you can passively listen to the line without the alarm knowing. You could pop the cap off the jack and clip onto the enclosed terminals. If you're good with electronics, you could build one.


What exactly do you mean by "pop the cap off the jack"?

If that's not an option, you're not out of luck. Assuming you have other phones connected to the line used by your alarm, you could also monitor the phone line through one of these phones. This works best with telephones that do not have an AC power source: old Western Electric or Automatic Electric phones are perfect. If you connect a suction-cup recording patch to the base of the phone you will be able to hear the phone line without picking up the phone.


How do those recording devices work without having an electrical connection? And if it seizes the line, wouldn't that prevent it from recording from another phone?

Also, if you have a VOIP adapter (like a PAP2), you could connect it to the alarm, let it dial out, and view the last dialed number in the web interface.


Good idea, but I don't have a VOIP adapter, and even if I did, I probably couldn't connect the alarm (with the wider plug I mentioned) to the adapter.

#5 nyphonejacks

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Posted 16 January 2012 - 07:53 PM


If you don't have any other phones connected to the line the alarm uses, you'll need a lineman's headset. When you have the thing in "Monitor" mode, you can passively listen to the line without the alarm knowing. You could pop the cap off the jack and clip onto the enclosed terminals. If you're good with electronics, you could build one.


What exactly do you mean by "pop the cap off the jack"?

If that's not an option, you're not out of luck. Assuming you have other phones connected to the line used by your alarm, you could also monitor the phone line through one of these phones. This works best with telephones that do not have an AC power source: old Western Electric or Automatic Electric phones are perfect. If you connect a suction-cup recording patch to the base of the phone you will be able to hear the phone line without picking up the phone.


How do those recording devices work without having an electrical connection? And if it seizes the line, wouldn't that prevent it from recording from another phone?

Also, if you have a VOIP adapter (like a PAP2), you could connect it to the alarm, let it dial out, and view the last dialed number in the web interface.


Good idea, but I don't have a VOIP adapter, and even if I did, I probably couldn't connect the alarm (with the wider plug I mentioned) to the adapter.


get yourself another phone jack... open the cover for this one and connect the green/red terminals for your new jack to the green/red terminals for this jack... then you can connect a DTMF decoder to the jack that you just added...

if for some reason the jack might be wired backwards, then the blue/white terminals inside this jack would go to your green/red on your new jack

#6 flarn2006

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Posted 16 January 2012 - 08:39 PM



If you don't have any other phones connected to the line the alarm uses, you'll need a lineman's headset. When you have the thing in "Monitor" mode, you can passively listen to the line without the alarm knowing. You could pop the cap off the jack and clip onto the enclosed terminals. If you're good with electronics, you could build one.


What exactly do you mean by "pop the cap off the jack"?

If that's not an option, you're not out of luck. Assuming you have other phones connected to the line used by your alarm, you could also monitor the phone line through one of these phones. This works best with telephones that do not have an AC power source: old Western Electric or Automatic Electric phones are perfect. If you connect a suction-cup recording patch to the base of the phone you will be able to hear the phone line without picking up the phone.


How do those recording devices work without having an electrical connection? And if it seizes the line, wouldn't that prevent it from recording from another phone?

Also, if you have a VOIP adapter (like a PAP2), you could connect it to the alarm, let it dial out, and view the last dialed number in the web interface.


Good idea, but I don't have a VOIP adapter, and even if I did, I probably couldn't connect the alarm (with the wider plug I mentioned) to the adapter.


get yourself another phone jack... open the cover for this one and connect the green/red terminals for your new jack to the green/red terminals for this jack... then you can connect a DTMF decoder to the jack that you just added...

if for some reason the jack might be wired backwards, then the blue/white terminals inside this jack would go to your green/red on your new jack


I don't think my parents would let me remove the cover. :-\ Anyway, would that suction cup thing work if it seizes the line?

Edited by flarn2006, 16 January 2012 - 08:39 PM.


#7 nyphonejacks

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Posted 16 January 2012 - 09:52 PM

you mean an inductive pick up coil? like this http://www.radioshac...oductId=2103876

possibly, but i doubt it...

1: how would they know you opened the jack if you put everything back the way that it was?

2: if you do not want to fuxor with it there, then just bring yourself out to the NID with a beigebox..

#8 JmanA9

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Posted 16 January 2012 - 11:53 PM

I don't think my parents would let me remove the cover. :-\ Anyway, would that suction cup thing work if it seizes the line?

The suction cup doesn't sieze the line. I'm not clear on the electrical principles behind this, but I believe what's happening is that the telephone generates electromagnetic leakage at all times. You won't be seizing the line because this principle is in effect even when the phone is hung up.

You can usually pop the cap off those jacks by inserting a flat headed screwdriver into one of the ends. There's usually a tab meant specifically for this purpose.

#9 ticom

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Posted 29 January 2012 - 01:34 PM

That phone jack is designed to allow the alarm system to seize the line, in order to prevent someone from disabling the system by picking up a phone in the house. You need to install your DTMF decoder (and hope the alarm system doesn't use pulse dialing) between that jack and the phone line's demarc point. Actually, the demarc point is probably the easiest place to access the line for experimentation.




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