[asterisk-users] FAX CNG detected but no fax extension

kevin.larsen at pioneerballoon.com kevin.larsen at pioneerballoon.com
Tue Nov 29 09:24:07 CST 2016

> Hello,
> I have a question regarding incoming fax to local file (on the 
> Asterisk server).
> While the fax is received properly (I have the tiff file generated 
> as expected) I get the warning 'FAX CNG detected but no fax 
> extension' on the consol.
> If the fax is received ok then what 'fax extension' does it expect 
> and what should I do there? 
> My Setup:
> Sender -> Public PSTN -> provider -> SIP trunk (configured with 
> G711a) -> Asterisk (13.6.0)
> My extension.conf on relevant section is this (obviously this is not
> production code):
> exten => s,1,Answer()
> same =>      n,Verbose(0, Attempt to Receive FAX)
> same =>      n,Set(FAXOPT(gateway)=no)
> same =>      n,ReceiveFax(/var/workspace/testfax.tiff,d)
> same =>      n,Hangup()
> and 
> Server*CLI> module show like fax
> Module                         Description 
> Use Count  Status      Support Level
> res_fax.so                     Generic FAX Applications 
> 1          Running              core
> res_fax_spandsp.so             Spandsp G.711 and T.38 FAX 
> Technologies  0          Running          extended
> 2 modules loaded

The good news is you don't really have anything wrong and as things are 
working as expected, you can ignore the warning if you so choose.

What generates that error is that on your trunk, you have faxdetect=yes. 
This will cause Asterisk to listen in to all your calls on that trunk and 
try to detect a fax and if it finds it will redirect it to a fax extension 
to be handled as a fax.

You have written a fax handler for your fax lines, but that doesn't stop 
the fax detection from trying to route it to an extension called fax. 
Since this doesn't exist in your case, you get the warning, but the fax is 
received because you are handling in the current path.

Where things would actually break is if someone sent a fax to one of your 
voice lines. If you don't have a fax extension to send it to, the person 
being called would pick up to fax tones. If you do have a fax extension, 
they would get the call yanked from them and it would be sent over to the 
fax extension. In my particular case, testing shows I get about half a 
ring to my desk phone before the system determines fax call and sends it 
to the fax system.

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