Some carriers gateways doesn't like a T.38 invite on advance, usually the order that gives better results for faxing are first invite on PCMU/PCMA (or any other supported codec) and then re-invite to T.38.

Take a look at how gonicus gofaxip do-it (https://github.com/gonicus/gofaxip)

On Fri, Dec 1, 2017 at 10:21 PM, Kevin Olbrich <ko@sv01.de> wrote:
Hi!

Our carrier supports T.38 for outgoing faxes. I am trying to send faxes like this:

originate {origination_caller_id_name='kevin',origination_caller_id_number='+49123456789',ignore_early_media=true,absolute_codec_string='PCMA,PCMU',fax_use_ecm=true,fax_enable_t38=true,fax_enable_t38_request=true}sofia/gateway/carrier_1/+4912121212 &txfax(/opt/faxout.tif)


Log:

668fd128-f2f5-4af5-9536-6780762c2efa 2017-12-01 21:38:57.513164 [DEBUG] sofia_glue.c:1295 sofia/external/+4912121212 sending invite version: 1.6.19 -36-7a77e0b 64bit
668fd128-f2f5-4af5-9536-6780762c2efa Local SDP:
668fd128-f2f5-4af5-9536-6780762c2efa v=0
668fd128-f2f5-4af5-9536-6780762c2efa o=FreeSWITCH 1512130268 1512130270 IN IP4 123.123.123.123
668fd128-f2f5-4af5-9536-6780762c2efa s=FreeSWITCH
668fd128-f2f5-4af5-9536-6780762c2efa c=IN IP4 123.123.123.123
668fd128-f2f5-4af5-9536-6780762c2efa t=0 0
668fd128-f2f5-4af5-9536-6780762c2efa m=image 30466 udptl t38
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxVersion:0
668fd128-f2f5-4af5-9536-6780762c2efa a=T38MaxBitRate:14400
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxFillBitRemoval
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxRateManagement:transferredTCF
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxMaxBuffer:2000
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxMaxDatagram:400
668fd128-f2f5-4af5-9536-6780762c2efa a=T38FaxUdpEC:t38UDPRedundancy
668fd128-f2f5-4af5-9536-6780762c2efa
668fd128-f2f5-4af5-9536-6780762c2efa 2017-12-01 21:38:57.533158 [DEBUG] sofia.c:7084 Channel sofia/external/+4912121212 entering state [calling][0]
668fd128-f2f5-4af5-9536-6780762c2efa 2017-12-01 21:38:57.673120 [DEBUG] sofia.c:6294 sofia/external/+4912121212 T38 invite failed
668fd128-f2f5-4af5-9536-6780762c2efa 2017-12-01 21:38:57.673120 [DEBUG] sofia.c:7077 Channel sofia/external/+4912121212 skipping state [ready][488]


Carrier confirmed that T.38 is possible... What could be the cause for this? How can I enforce T.38 instead of falling back to g711a (hangup on failed invite)?

Thanks.

Kind regards,
Kevin

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