VOIP to Analog Intercom (Grandstream Phone)

Hello All,

Background: Replacing Mitel Analog phone system with Grandstream GXP1450 phones and PBXinaFlash/FreePBX system, (for a school district).

PbxInaFlash Kernel Version: 2.6.32-358.6.2.el6.i686 (SMP) i686
FreePBX version:
Grandstream VOIP Phones: GXP1450

Issue: I am trying to tie our analog intercom (speakers around the building) to our VOIP system. There is an analog command phone that when you off hook the phone you’re already directly connected to the command console, no ringing or dialing necessary. If you dial #10 (for Simplex Building Communications Systems) it will ding and you can talk over the intercom.

Ideal Situation: Ideally you would be able to dial an extension on a Grandstream phone and ding you could talk over the intercom. Users on the analog system currently dial an extension and are able to do this.

Things Attempted:

  1. Using a Handy Tone ATA (Grandstream HT701) to interface between a ethernet connection/voip extension and analog phone. Problem is, the Simplex system doesn’t detect that its ringing when calling so the ATA never allows the call through to the command console, (status on ATA just says Ringing and never changes to off hook).

  2. Set up a Grandstream GXP1450 phone set to auto answer, set the headset toggle to on, and pressed the headset button on the phone. I then ran a “RJ9” cable (4P4C) from the headset port on the phone to an RJ11 jack on the wall connected to the Simplex system. (This is previously where the analog phone was connected for direct access to the command console). Made many attempts by dialing the extension of the 1450, which picked up and went straight to the headset port. I dial #10 and nothing happens, no ding and talking into the line doesn’t matter. I have attempted to change the DTMF settings on both the calling and receiving extensions from RFC to inbound. I have set DTMF settings on the phones themselves as well. I cannot figure out what is going on.

Extra Info:
So I am wondering if I have DTMF settings that are preventing the command console from “hearing” the #10 buttons from being pushed, or if there is a command preventing the pound “#” key from passing through or what is going on.

It would appear that my second setup is as close as humanly possible to get to what I need to do. Are there any other settings anyone can think of that I could adjust? Is there a better way to do this?

If you offer log request please include how to do this as I am a complete novice when it comes to Asterisk/FreePBX.

Thank you to anyone that can help or point me in a direction to test!

Just a few things to think about, really can’t get more specific without more details on the intercom system.

First of all, your “command” phone is may not be a “phone” in the conventional sense. From your description, it could use 3 pair, the first provides a contact closure to the system, and the other two are TX audio and RX audio.

Second…If it is just a regular analog phone, your scheme won’t work. The jack on the wall is an FXS device, and the jack on the ATA is also an FXS. An FXO has to connect to an FXS.

see: http://www.3cx.com/faqs/FXS-FXO/

You might be able to use an FXO gateway like the Linksys SPA3102, but without more info on your paging system itself I’m just guessing.

Another option might be to locate the audio input to the audio power amp and tie in something like the ALGO 8180 SIP Audio Alerter.

These are just some random thoughts, and as I said, just really guesswork.


I believe it is just a standard analog phone. It isn’t a special command phone with additional keys. I can drop any analog phone on that line and it works properly. I’ve also replaced the phone cable.

The Linksys device you reference - I believe I was looking at this before. When I went to purchase it I found a lot of reference to “unlocking” the device to be used with other carriers/equipment. Are you familiar with it at all? Is this an issue?


Tou’re going to have to purchase an unlocked version which should be readily available from a number of sources.

Additionally, is there an AUX input available. Many units of this type have a way of inputting an external audio source. If you can get to that, one of the ALGO units I described would let you utilize that…