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Trouble with VG224 and Fire Alarm modem Dialers

Hi folks, got a situation here that maybe someone has ran across.

Just recently we have switched over to 100% VoIP and got rid of our PBX system. We now have our old fire alarms running on VG224's which are for the most part running fine. These have been in existance for many many years. We had some issues with some of them in which we found that by switching a VG to a h.323 gateway solved the problem for us.

Using MGCP as well

As a growing University we are constintaly adding in new bldgs which means we are adding new Fire alarms. With each new fire alarm we add in, it seems that the dialers do not want to work with our VG's. The dialers are set at 2400 baud.

Has anyone ran across this issue before?

We are using CUCM

-Thanks,  Nick


Being from the PBX world I have seen similar problems with modems, faxes, etc. Normally involving devices using CNG type tones. It almost always turned out to be a compression issue on the line. In your case this would be the VG24.

We have had PBX's running with internal analog devices connected for years with no issues. Then the customer would sign up for a voice over IP service from their voice carrier. This voice over IP service would use a compression setting of some sort that inevitably caused the fax or modem device to fail. We were always able to verify the problem by placing the device that was failing back onto a copper trunk and monitor it. It worked every time.

I realize this does not fix the problem for you, but it might be a good starting point. If you have a copper line available place it on one of the alarms and test. If it works then you know you need to start looking at the VG24 and the IP connection.

Last, though not my business, you may want to re-consider having the fire alarms going through the phone system.  Some of the localities around us do not allow this purely for life safety issues.


We do have copper lines to verify which is why I am here asking for help with my VG configs Cass... hehehe.

We have not had any issues with fax lines or modems, just fire alarm dialer modems.

When the modem dials the other end... in this case the master fire alarm server to "check in" so to speak, the handshake takes place and connection is established, however when the data flow starts something is happening and the call is being dropped. We were thinking perhaps DTMF tones on the voice_ports (timing digit, timing inter-digit) or something else...


Did you every figure anything out regarding the VGs working with fire alarms? 



I would also be interested if you made any progresss with problem or found any soulutions.

We recentntly converted a campus over from anothe voip/pbx solution to coisco voip.

I am using vg224s and and a fxs card on a router.




It’s possible your alarms use the Ademco Contact ID protocol - a DTMF based protocol (rather than using modem tones) used to communicate over the PSTN (eg. back to base monitoring). The DTMF tones are very short duration – typically in the range of 40-60msec. On a conventional TDM telephone system, this does not cause any issues as the network passes any tones and does not care about the actual duration of the tones.

On a VoIP system with DTMF relay in place the DSP may detect these tones but discard them because they are considered too short, meaning no DTMF tones will be relayed out of band and the alarm call will fail.

We’ve had success using g.711 and disabling DTMF relay so the tones stay inband, we’ve done this with a VG224 with h.323 dial-peers pointed to another h.323 gateway (29xx router) with a PRI.



Thanks for your reply. I confirmed with the Technician that the pannels do use "Contact ID". We are moving the line back to the old tmd pbx system temporarly untill we can get a working configuration and any additional equipment we need to support passing the DTMF tones across the network to our LD PRIs. Thanks for your input. I was able to find some documentation on the protocol:

Digital Communication Standard - Ademco ® Contact ID Protocol

I am glad to here that you and other have found workable solution for your enviorments. We will be working with a partner to design a scalable solution as that remote campuse will be adding more buildings in the future.


   Hello, Jonathan.

  I am more of an end user than an IT person.

  Could you help to explain how this would be entered? (I'm sorry, but I am trying to pass this on to IT as they can't seem to figure it out.) They are currently using MGCP, I am told. (I think you can used different Protocals on different Ports, but I don't understand exactly how.)

  I need 2 Lines to work with Alarm Dialers which have short duration DTMF Signals.

  Thanks for any help.




We end up having to convert our MGCP gateway (a cisco router) to h323 and connecting the alarm lines directly to it.

The issue is that the dtmf tone must be kept inband and in some case could still get croupted across the network.

The is why we converted our gateway which the pstn connects to to h323. Basicly this allowed us to keep a tdm to tdm connection without sending anything over the network.

It doesn't just matter that the port with the alarms is configured for h323, the connection to the pstn would also have to be h323. If not it will still fail.

I am not infavor mixing the protocols for ports on a gateway. I would rather have a seprate gateway for alarms from other devices such analog service (plain old voice), fax and modems.


Very useful post. I am in the same perdicament. I have firealarms of multiple flavors, secure phones and modems. The only thing working is the firewalarms becasue they made the ISR a h232. But I cannot get my modems and secure phones to work because they love mgcp. My 2 ISR's serve as the ISDN T-1 backbone and 3 vg's serve as my analog lines for these systems. 

Hall of Fame Community Legend

Hi Jonathan,

Very,very helpful answer here +5 for this great info.




We have corporate networks with hundreds of fire and security panels using modem II and modem 3 protocol. Reducing phone lines and deploying SPA112 to provide VoIP lines to each alarm panel. Has anyone had success without call droppings on this modem 2/3 protocol? Which settings were favorable?

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