T38 gateway / asterisk 11

I am trying to setup an asterisk server as a T38 gateway. Here is the schema:

(telco sip) <—g711–> asterisk 11.25 <----g711—> ipbx

When I use G711 between asterisk and the ipbx, it works. But I want t38 between asterisk and the ipbx. When the IPBX receives a fax in g711 and issues an (re)INVITE for T38, I get an error 488 NOT ACCEPTABLE HERE replied by asterisk 11.25.

For testing purpose, I’ve setup:
(telco sip) <—g711–> asterisk 11.25 <----t38—> asterisk 11.21
same issue

and I get the error on the asterisk 11.25:
[Jan 3 22:43:20] ERROR[21508][C-00000003]: res_fax.c:1072 fax_session_new: Could not locate a FAX technology module with capabilities (V21)
[Jan 3 22:43:20] ERROR[21508][C-00000003]: res_fax.c:2569 fax_gateway_new: Can’t create V21 session on chan SIP/telcotrunk-00000006 for T.38 gateway session
[Jan 3 22:43:20] ERROR[21508][C-00000003]: res_fax.c:4108 acf_faxopt_write: Error attaching T.38 gateway to channel SIP/telcotrunk-00000006.

Why is that ?
Thanks for the help

From the CLI on the Asterisk 11.25 machine execute a

asterisk -rx “core show applications like fax”

and post your results - from the line above, I think you don’t have SPANDSP Fax compiled/loaded.

This is what it should look like:

[root@asterisk ~]# asterisk -rx “core show applications like fax”
-= Matching Asterisk Applications =-
ReceiveFAX: Receive a FAX and save as a TIFF/F file.
SendFAX: Sends a specified TIFF/F file as a FAX.
-= 2 Applications Matching =-

Info from the Digium guys and gals specific to your question:


thanks for the reply - the boxes have Digium FFA with the free licence.

I think that is the problem for what you are trying to do - Gateway a T.38 session - the FAQ is here: http://my.digium.com/en/docs/FAX/fax_faq/ but from everything I have read, FFA is more of a termination or origination - not a Gateway solution.

Perhaps a Digium person can weigh in here, but I think to do what you want, you will need the SpanDSP FAXing to make this work - getting it compiled into Asterisk is not at all hard.

That is correct, FFA does not support the required functionality for gatewaying. Only the spandsp driver has it.

thanks - I was convinced it did.

Just to be clear, as I have done quite a few tests…

FFA requires res_fax.so and res_fax_digium.so
and spandsp requires the same res_fax.so and res_fax_spandsp.so

is this correct ?

Yes, that is correct.

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Asterisk can’t translate from g711 to t.38, and from t.38 to g.711. If You want to do it, You’ll have to use gateway functionality in dialplan or in sip.conf.