How To Enable T.38 Between SPA3102 PSTN Line Port And ReceiveFAX App ?

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Asterisk Users 3 Comments


I’ve got an analog phone which is currently receiving unsollicited FAX
calls from PSTN.

For learning purpose, I’m preparing an Asterisk/SPA3102 setup that would let voice calls come in and out and translate incoming FAX calls to TIF
files (forwarded through email)).

My target setup is :

PSTN <-- analog--> SPA3102 Line Port <-- SIP --> Asterisk <-- SIP -->
SPA3102 Phone Port <-- analog --> Analog phone

When a call comes in, analog phone rings. If callee answers and a fax tone is detected, then the incoming call is sent by Asterisk to ReceiveFAX application which translates incoming audio to TIF file.

My setup is working ok when I’m using ReceiveFAX in fallback mode (with f option).

Then I would like to improve my setup letting ReceiveFAX negociate T.38
with SPA3102. The trouble is SPA3102, as I configured it, seems to refuse T.38
negociation as I’m reading lines like this in Asterisk logs:

== Using UDPTL CoS mark 5
[2013-11-05 10:36:50] WARNING[3061][C-00000007]: res_fax.c:1698
receivefax_t38_init: channel ‘SIP/myline-0000000e’ refused to negotiate T.38

My question is:
Any hint on how to configure SPA3102 PSTN Line port so that it would accept to upgrade to T.38 ?

When Asterisk re-invites SPA3102, here is the dialog between both boxes:

INVITE sip:myline@ SIP/2.0
Via: SIP/2.0/UDP;branch=z9hG4bK260a479e Max-Forwards: 70
From: ;tag=as1a0daffe To: myline ;tag-2e7e5ad74dec0co1
Call-ID: c8278ef2-b80a5454@
CSeq: 102 INVITE
User-Agent: FPBX-2.11.0(11.3.0)
Supported: replaces, timer X-asterisk-Info: SIP re-invite (External RTP bridge)
Content-Type: application/sdp Content-Length: 257

o=root 473469291 473469292 IN IP4
s=Asterisk PBX 11.5.0
c=IN IP4
t=0 0
m=image 4506 udptl t38

3 thoughts on - How To Enable T.38 Between SPA3102 PSTN Line Port And ReceiveFAX App ?

  • The SPA3102 only supports T.38 on the FXS port, the FXO port uses G711
    for a fax session.

    The Grandstream HT503 supports T.38 on both the FXO and FXS ports.

    What problem do you have receiving a fax over G711?


  • 2013/11/5 Larry Moore

    That explains why I couldn’t find the option 😉

    I never tried this one. How would you rate this product ?

    Is it easy to auto-provision an HT503 over TFTP or HTTP ?
    Is it easy to localize FXO/FXS setttings for non-US countries (those having played with a SPA3102 sure know what I’m thinking about) ?
    Do it T.38 implementation works ok with Asterisk ?

    I tried few calls in G711 fax mode and it worked OK but I rated T.38 as more reliable solution in the long run and in general.

  • .

    I don’t use it for faxing only for the purposes of testing its capabilities receiving. I have progressively upgraded the firmware as it has been released.

    It has these options though I have not used auto-provisioning with it.

    Grandstream have configuration tools and templates, see

    In short, Yes!

    Seems to though I really have only performed basic testing receiving a fax through it from the PSTN (FXO port).