Fax Pro - Not working Inbound or Outbound

I am having issued with receiving both inbound and outbound faxes.

For a benchmark test. I set up a test extension #990 and sent a fax from 990 to 990. SUCCESS!.

Then I tried to send a fax from eFax to FreePbx.

There are multiple errors but the biggest error (several hundred) is this one. Buffer overrun. I figured we could start troubleshooting with the most prominent error and work from there.

13894	[2025-02-02 18:00:33] ERROR[10589][C-00000030] udptl.c: UDPTL (PJSIP/CA-Sacramento-HWY50-00000025): Buffer overflow detected (3 + 3 > 3)

Anyone have any ideas of where I could start troubleshooting?

Silly question, but udptl is enabled in your trunk ?
It should be in the trunk configuration, under pjsip settings - advanced.

Thats not a silly question at all. I skipped past all the fun of the 1980s and this is the first FreePbx Fax service I’ve needed to setup.

Here are the UDPTL trunk settings

Try adding 400 as the maxdatagram value

I think that solved the buffer error.

Now the next noticeable item in the log is these two items;

24232	[2025-02-02 19:19:25] NOTICE[459][C-00000046] res_fax.c: FAX T38 detected but no fax extension in context (ext-fax)	
24233	[2025-02-02 19:19:30] WARNING[459][C-00000046] res_fax.c: channel 'PJSIP/CA-Sacramento-HWY50-00000050' timed-out during the T.38 negotiation.

Inside your inbound routes - fax, is it enabled and do you have a destination going to whatever extension you want to use to receive faxes?
I can follow this tomorrow, too late in Argentina (00:30) :smiley:

I got inbound faxes working! Your suggestion led me to the right issue.

I had to turn of “Detect Faxes” on the inbound route.

It seems FreePbx has a few places along the path where a fax can be detected, and this particular spot needed to be turned off.

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