Faxing and T38

Well, this would seem a like a fairly simple task, but no luck.
I use flowroute as my provider, and at this time I am allowing all traffic from their IP addresses. So surely port forwarding is not an issue.

I have performed the following configuration items via the freepbx gui:

And I have performed a tcpdump on my asterisk server’s interface:faxtest.pcap

(although I don’t know what to look for)

Outbound faxes through an ata and traditional fax machine work fine. But inboud faxes are not functioning correctly. The pbx seems to detect the fax, since it does not go the IVR, but the fax does not complete. Nor do I hear any fax tones being sent back from the pbx system. Finally, in the CDR the call shows the term ‘hangup’ under ‘App’ and ‘s [ext-fax]’ under ‘Destination’.
There seem to be a lot of T38 packets in the pcap, so I assume T.38 negotiation is proceding correctly, although the IP address the server is attempting to send the packets back to seems odd (

Any advice?

A virtual fax is a system fax machine that will email you the fax. You need to setup a regular extension for the ATA as the fax destination.

You did not mention how FreePBX was installed. Asterisk and FreePBX versions and ATA type.

I appreciate the response skyking.

I actually intend on receiving faxes on the virtual fax extension, but sending them via a traditional fax machine on a generic sip extension.

Otherwise I used a PIAF install, I believe it was green. So it is Asterisk 11.5.0 and FreePBX The ATA is a Grandstream HT701.

I don’t know what fax receiver PBX in a flash includes. Our Distro uses SPAN DSP.

You will have to check with their forums.

Alright, I’ll give them a shot.

After doing some tcpdumps on the pbx and router, there appeared to be a change to the SIP Invite packet between the pbx’s interface and router’s wan interface. This change modified the ‘IN IP4’ part of the ‘Owner/creator’ field in the message body. It changed the IP address to some other incorrect address.

Why this change occurred I could not figure out until seeking support from my provider (flowroute), and getting their assistance. Their suggestion was to disable SIP ALG on my router, and sure enough, this solved the problem.

Like that isn’t documented at least 1000 times.

Always disable any type of SIP transform in your router/firewall.

I apologize if you found my last post annoying.
I was simply trying to describe the exact problem I was experiencing and the solution for it. Hopefully, other people who search t38 on these forums can test their setup for the same problem, and then attempt the same fix – just in case they had not already done so.
I’m quite sure this is considered good forum etiquette, I hope it does not cause you too much discomfort.

It’s not discomfort at all. For years we have posted how to setup networking and SIP. The first advice is always to disable router/firewall based transforms.

What is annoying is that no matter how many times we post, update wikis and other documentation the same questions keep coming up in the forum.

Why won’t people slow down and read the docs?

As we often say, it’s in the wiki, in kimbokasteniv’s case twice,




But Scott, why would anybody bother to read the wiki when they can get their answer without bothering to do even the most basic due diligence? . . .

So keep on guys (and girls) just type away blindly, maybe you will get lucky one day.

(had our friend bothered to read the wiki, in his case he would have saved himself nearly three weeks of wandering around in the mire, come on guys , just read the wiki before you ask . . .)

For some reason people seem to think we are withholding answers. It’s very odd. I had a customer a few weeks ago. I think he spent his last $150 to have me fix his FreePBX system (I didn’t take his money). I sat down and after about 10 minutes he was surprised that I had no clue what was wrong. When I started searching online I think he thought I was a fraud.

Most of the time we (meaning the somewhat more experienced folks who hangout here) have to go look up the answer.

We want this project to continue to be the success that it is, wouldn’t do anything to hurt it. If you do your part we will bend over backwards to help you.

Skyking, if you feel that I had implied that the reason for my trouble was somehow related to you, then I apologize for that. That was definitely not my intention nor the case.

Anyway, as you guys have described, this is a clear case where I missed something in the wiki. I had read a number of posts on here and elsewhere, and I am afraid I got tunnel vision on T38. I did consult the wiki here, but I simply searched ‘fax’ and ‘t38’ and read the information directly related to it, and for all I know ALG may have been mentioned there as well. But I mistakenly assumed that since voice calls were working as intended the problem must be directly related to some T38 issue. Thus I did not read any basic network configuration info here.
This is a classic case of making one assumption which was ultimately incorrect, and a dumb mistake.

With all of that said though, posts of this nature are extremely common on any forum. Typically they are far worse, since they will be composed of nothing more than “not working, why?” Then the new user will get irate when others fail to help, and etc. I can understand the frustration.
I tried my best to provide as much relevant information in my original post. I remained polite. And as far as I can tell, I did not blame anyone for my failing. When this problem was finally resolved, I came back and posted a solution for others. I did, however, make the oldest mistake in the book.
After all of that though, the part that surprises me is that the more veteran users actually insulted a user for acting this way. That’s really unfortunate. No doubt did I miss something in the wiki, but that’s the typical scenario for many posts.

I can understand how frustrating it must be to maintain a wiki only to have many users miss the content of it over and over. But I am afraid this is how it has always been and always will be. I hope in the future posting on these forums becomes a more pleasant experience for you guys.

Again, I apologize for my mistake.

From your post:-

. . . After all of that though, the part that surprises me is that the more veteran users actually insulted a user for acting this way. That’s really unfortunate. No doubt did I miss something in the wiki, but that’s the typical scenario for many posts.

I can assure you there was no intent to insult you , just to chide you and all the others for not doing your homework. You are however absolutely right , you did miss something, and yes that is typical in many,many posts here, our intent is to reduce the occurrence of such willful negligences. Many folks work hard to build and maintain that body of information, I implore you and all others to actually spend some time reading through it, and I mean all of it and more than once if necessary, it will really save an enormous amount of time for all who use this forum and would for us indeed be a pleasant surprise.

As Dicko pointed out, it’s not a personal insult. It’s a matter of time and effectiveness, frustration and desire to communicate effectively.

The other truth is, the internals of FreePBX are increasing in complexity geometrically. Hacking on it and 3rd part developer support is far more challenging.