Fax detect help

I am having a difficult time getting SIP fax detect to work. We use PBX in a Flash with Asterisk and FreePBX here in our office. We have an all SIP environment. We have Aastra phones, an SPA2102 for two analog fax machines, and an SPA3102 for my home.

Dialing 666 from an analog fax works as expected (pbx answers and emails me the page).
Setting 666 as a misc destination then tying that to an inbound trunk works (from an outside fax).
Setting an extension (a regular SIP and virtual) to be fax capable and putting SIP fax detect on the inbound route (and putting the destination as the fax enabled extension) didn’t work.

I tried dahdi and sip and checked the config files for the appropriate entries. I came across an entry at voip-info.org that showed a dialplan that had fax detection in it. I decided to look at extensions_additional.config and found that the IVRs had an entry that referenced faxing.

Out of curiosity, on the inbound route, I set the destination to an IVR (while still keeping the fax detect option set to a fax enabled extension) and it works. When listening on the analog fax machines speaker, the IVR picks up the call before the negotiation process. This is telling me that the call has to go through that IVR dial plan before the fax detect works. I set the fax detect time from 2-10 detection time with no change. It was always when the IVR picks up that it was able to detect. So, it would seem to me that whatever is done with the call during that fax detect time period, there is some error.

I figured I could setup up a dummy IVR to be the fax detect time, but having it work with the actual setting would be great, I just don’t know how or where to look now.


With Asterisk and Freepbx, I haven’t had the best luck with the detection working correctly. If you want faxing to work, set up a dedicated extension using 100% ulaw - as you did. I’ve had the best results with auto detection while using a t38 license installed and a sip trunk provider that supports t38. I believe the license had a one time cost of $10 but it has made things much easier. Downside is that one sip channel had to be purchased and dedicated to t38.

If you use g729, be careful to not use it on the sip trunks with incoming faxes.

What I’ve done is setup a dummy IVR that the call goes to first. Odd thing, if there is no join announcement, it doesn’t work. So I’ve recorded a ringing tone and placed that as the join announcement. After that it rings the extension. I’ve had 100% success rate on detection.

To recap, the inbound route has detect faxes enabled. Wait time is the minimum selected (2). The fax recipient is a regular sip (voice) extension that has faxes enabled. The destination is the dummy IVR. The IVR had a ringing tone (one full ring and the dead time following - total time - just under 6 seconds). The timeout on the IVR is zero and t destination is set on the IVR to failover to the voice extension (happens to be the same extension as the fax recipient above).

Behavior is when a voice call comes in, the calling party has a slight 2 second delay before they start hearing a ring. After the first ring they hear, the extension start ringing.

When a fax comes in, it is detected in that first ring and handled appropriately.

To get the ring tone, I just set a misc application to go to terminate - ring forever then recorded the call (I put my phone on mute). I then edited the file in audacity for the length I wanted.

Hope this helps someone else.


Why dont you just up the detection time to 6 seconds?

With the detection time up to 10, it still did not detect. In fact, with my ‘IVR detect’ setup, I can put the FreePBX detection time up to 10, and the CNG fax tone will not be detected throughout those 10 seconds but as soon as the IVR answers and the CNG tone happens, it detects it immediately.


I have this exact issue… but from what I can tell it’s isolated to PBX in a flash… I installed FREEPBX Distro and it worked as it should… Not sure why this would be because both distros use the same Asterisk versions and same freepbx versions… accept perhaps a different edit for PBX IN A FLASH.


Feel free to use the FreePBX Distro.
If it is your name, your business, your reputation it seems to make sense to come where we “have a huge amount of PAID staff who work on fpbx and who also donate their time, and they have more resources…” Rather than telling you “have a nice day we will get around to fixing it when we can. There is no ETA as we have a couple of other larger higher priority problems that need our immediate attention versus dropping all of our development time to concentrate on a rather small issue although I am sure you consider it of the highest priority”

It seems that is their desire considering the closing from their lead developer “I am happy you found another distro who serves your needs I wish you well with it and adios.”