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Inbound T.38 Fax consistently failing

Discussion in '3CX Phone System - General' started by Legop, Mar 21, 2018.

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  1. Legop

    Feb 22, 2018
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    Hello, I would like to receive faxes on a single DID and have them sent by email.
    The integrated "3CX Fax Server" ist designed to do just this.
    It's configured like this(some data obscured):

    There's an inbound rule auto created, sending all calls on that DID to 888, at all times of day.
    When receiving faxes with T.38, detailed logs in 3CX show these two errors randomly alternating:
    BYE sip:[obfuscated]@ SIP/2.0 Reason: SIP;text=User Hung Up
    SIP/2.0 488 Not Acceptable here Reason: SIP;text=Application Rejected Sdp(usually no common codec)

    Receiving G.711 faxes works ok.

    3CX is running on Debian 9.2.1 with version 15.5 latest updates applied.
    It's behind a Unifi Security Gateway. The firewall test passes OK. SIP Conntrack dis/enabled has no effect.
    SIP Trunk provider is easybell. They fully support T.38.

    I've tried unchecking the checkbox for the G.711 Fallback to T.38 (which is documented to listen for fax tones for 10 seconds and if none received, switches to T.38). This results in the error:
    SIP/2.0 480 Temporarily Unavailable

    No faxes that are sent with T.38 are being received.

    Easybell Support cannot find any fault on their side.

    Further tests included using a Fritzbox as a SIP Client, configured as a fax extension, with similar issues.
    Restarting the entire PBX didn't change anything.
    There's no problem with regular calls in- and outbound.

    A pcap shows malformed packets after switching to T.38 when the INVITE is sent:


    Any ideas highly appreciated.
  2. YiannisH_3CX

    YiannisH_3CX Support Team
    Staff Member 3CX Support

    May 10, 2016
    Likes Received:
    Hello @Legop

    If you have a capture of an inbound Fax attempt please send me a p.m. so i can check. We might also need the support files of the PBX.
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