T.38 Faxing

FreePBX 2.9 distro
I have freepbx running in a production environment. We have recently been in the process of migrating our DID’s to a new SIP trunk provider. The provider supports T.38 faxing. With that said I was testing faxing today and I seem to be able to receive faxes from the outside world, but cannot send. When I send the distant fax rings, picks up and I hear CNG tones, the fax starts to send, but then fails. for what it is worth the FXS gateway is a grandstream 4004 gateway that supports T.38 also. I have been looking over the forums and I see a few that are discussing adding “t38pt_udptl=yes” to /etc/asterisk/sip_general_custom.conf

I probably don’t understand as much as I should about this, but I feel this has to do with extensions and not trunks. Might something like this need to be added to the SIP trunk from my provider?
Andy advise on settings and or debugging/ troubleshooting would be appreciated.
thank you in advance.

Sounds like to me a negotiation issue with your carrier. Most carriers do T38 inbound fine but cant guarantee outbound T38 as they send the call T38 and if the carrier who terminates the call can not translate the T38 the call fails. You need a carrier who guarantees delivery of outbound T38 by acting as a gateway and sending the fax to the PSTN as standard V29 faxing.

I know of no carriers who will do this so good luck

Tony- thank you for the reply. To go further on the subject, I did a lot of testing with this new SIP trunk early on and I had 100% success on sending and receiving faxes. I will talk to the carrier to reverify that they support in and out on the faxing, but we have discussed that in the past. I know that does not mean anything… :slight_smile:
On the FreePBX side are there no config changes that are necessary to get this to work or have more success?
I have been thinking of what might have changed since I tested this with the carriers SIP trunk and the only thing that I can think of is that the box that I have the fax extension on is connected via SIP trunk to another FreePBX and that freePBX is connected to the SIP provider. I will try setting up an extension on the FreePBX that is actually doing the trunking to the SIP provider.
For what it is worth the SIP trunk and the FreePBX are onnet in a data center so that leg of the call is not over the internet.
Any further advise or thoughts would be great.

I did more testing today and I have found that it has something to do with the SIP trunk that connects the two FreePBX together. I set up an extension on the BOX that connects to the provider SIP trunk and I can send and receive faxes without fail. I also for what it is worth discussed this with the provider and they support T.38 for inbound and outbound faxes.

To recap the topololigy
this does not work
SIP Trunk Provider---->FreePBX01-----SIP Trunk----->FreePBX02---->ATA
this works
SIP Trunk Provider---->FreePBX01----->ATA

any advise or help will be great.

I am getting a bit closer. I now have it to where the fax on the far end will answer, but the fax will still not go thru. getting closer.

I was watching BOX01 (the FreePBX that is connected to the SIP trunk) and saw

e[0K == Redirecting ‘SIP/MAIN_VCT_BOX02-00003403’ to fax extension due to CNG detection

I disabled the fax module in freepbx and that allowed the fax to actually get out of the FreePBX server and connect to the distant fax machine. Now I just need to figure out why I can not get a fax to work on this FreePBX (Box02), but I can from BOX01

If anyone has any thoughts that would be great.