Outbound T.38 Via RTP With Pjsip Does Not Work As Expected

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Asterisk Users 2 Comments

Hello!

I’m trying to send a fax via T.38 to a destination, which should be T.38
capable. My provider supports T.38, too. Unfortunately, it doesn’t work. This means:

Call is started and SDP is negotiated w/ alaw. Callee sends reinvite –
for alaw again (and not for T.38)!! After about 30s, callee hangs up because of missing data (this is true, because I don’t send alaw coded fax data.

Tracing the signaling shows, that the callee doesn’t have any possibility to recognize, if I’m supporting T.38, because it is never sent during Invite process.

I’m missing the media feature tag sip.fax in the contact header. Did I
miss some configuration?

That’s my setup:
Hylafax sends fax to t38modem and t38modem is connected via SIP to asterisk as extension. The extension is bound to an outbound route, which uses the t.38 capable ISP.

pjsip.endpoint.conf:

[ISP]
t38_udptl=yes t38_udptl_nat=no # there is no nat necessary t38_udptl_ec

2 thoughts on - Outbound T.38 Via RTP With Pjsip Does Not Work As Expected

  • This is not currently supported in either chan_sip or chan_pjsip. There’s no configuration which will enable it. It would need to be written. Have you confirmed this is what is needed by them?