T.38 Outbound Fax Failure - The Provider doesn't Fully Support T.38 Fax

  • Firmware version: Any
  • Model: S-Series
Problem Description
A SIP trunk is created for inbound and outbound fax on S PBX.
Inbound fax is working fine, but the outbound fax not.
Fax machine is connected to the FXS port of S PBX.
The PCAP log shows the T.38 fax is not initiated after Re-INVITE from provider.
Try to enable Error Correction on PBX, same issue.
Disable the T.38 Re-INVITE on provider side.
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  • 0

    Do we have a good example of FOIP flow traces?

  • 0

    The above solution is not practical as the sender cannot get access to the receivers settings to turn off "Disable send Re-Invite after fax tone detected" ( The receiver wants to preferentially transmit via T.38 which is not unreasonable thats why the receiver provides a T.38 RE-INVITE)

    The S PBX is not able respond to the Invite SDP (t38) from the receiver - why is this so? - is this a bug to be fixed in new firmware for S PBX? - happy to provide some fax endpoints for testing.

    A  solution is to disable T.38 in the S PBX settings for Outgoing Sip Trunk and then T.38 reinvite is ignored by the S PBX. However if you are using the same sip trunk for incoming you have completely disabled T38 for incoming and outgoing faxes which is very retrograde step.

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