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FAX to some numbers are not working

Discussion in '3CX Phone System - General' started by dustycr, Nov 9, 2017.

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  1. dustycr

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    Hello all,

    since two days our fax is not working but only when sending to specific numbers.
    Following the setup

    Printer / FAX Machine -> Grandstream HT701 -> 3CX

    Our provider is easybell which is not accepting t38 protocol, so it's disabled in the the grandstream options and set to passthrough mode.

    When a fax being sent to one of the not working numbers i can see in the activity log that 3cx sends the following t38 options

    Code:
    a=T38FaxVersion:0
    
    a=T38MaxBitRate:14400
    
    a=T38FaxRateManagement:transferredTCF
    
    a=T38FaxUdpEC:t38UDPRedundancy
    Aftet that im getting the following message:

    Code:
    Sending: OnSendReq Send Req BYE from 0.0.0.0:0 tid=38342050ba42355e Call-ID=211544261-5060-564@BA.BA.BA.CF:
    
    BYE sip:80@10.10.10.25:5060 SIP/2.0
    
    Via: SIP/2.0/ ;branch=z9hG4bK-524287-1---38342050ba42355e;rport
    
    Max-Forwards: 70
    
    Contact: <sip:xxxxxx>
    
    To: "FAX"<sip:80@10.10.10.1>;tag=1936181727
    
    From: <sip:xxxxxxxxx@10.10.10.1>;tag=39216126
    
    Call-ID: 211544261-5060-564@BA.BA.BA.CF
    
    CSeq: 3 BYE
    
    Reason: SIP;text=Application Rejected Sdp(usually no common codec)
    
    Content-Length: 0
    
    
    
     Offer is rejected on Leg L:1838.1[Fax:80];
    OnOfferRejected Recv 488/INVITE from 10.10.10.25:5060 tid=719cd542325bdd09 Call-ID=211544261-5060-564@BA.BA.BA.CF:
    
    SIP/2.0 488 Not Acceptable Here
    
    Via: SIP/2.0/UDP 10.10.10.1:5060;branch=z9hG4bK-524287-1---719cd542325bdd09;rport=5060
    
    To: "FAX" <sip:80@10.10.10.1>;tag=1936181727
    
    From: <sip:xxxxxxxx@10.10.10.1>;tag=39216126
    
    Call-ID: 211544261-5060-564@BA.BA.BA.CF
    
    CSeq: 2 INVITE
    
    Allow: INVITE, ACK, OPTIONS, CANCEL, BYE, SUBSCRIBE, NOTIFY, INFO, REFER, UPDATE
    
    Supported: replaces, path, timer, eventlist
    
    User-Agent: Grandstream HT701 1.0.8.2
    
    Warning: 304 GS "Media type not available"
    
    Content-Length: 0
    In working fax calls the t38 options are not sent.

    Has this problem something to do with the connection between voip and normal phone lines ?
     
  2. sip.bg

    sip.bg Active Member

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    No, I think you must use a provider supporting T.38.
    In other case, i.e. G.711 passthrough the results could be less than 50% success rate, depends on many factors like latency, jitter, packet loss.
    In settings | fax server try to enable G.711 fallback to see if any improvement happens
     
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  3. dustycr

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    For incoming faxes we dont have any problems its just outgoing to some numbers.
    Or does the fax server option G.711 fallback affect outgoing faxes too ?
     
  4. sip.bg

    sip.bg Active Member

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    Try it, I haven't found any documentation. The status is experimental.
    I think it affects only ATA used and not the SIP trunk.
    The PBX is transparent for T.38 operation - both SIP trunk and ATA must support T.38.
    If fallback is enabled, probably the adapter will be able to renegotiate T.38 to G.711 and vice versa.
    In FreePBX for example the most stable combination is T.38 capable trunk and G.711 ATA.

    Make a wireshark captures to investigate the reasons for failures and examine also the fax logs.
     
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  5. lneblett

    lneblett Well-Known Member

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  6. us1

    us1

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    Try bypassing the 3cx system and register the ATA directly with the SIP provider. This should help reduce a hop in the chain and further determine the source of the problem.
     
  7. dustycr

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    It was registered as a Fax Extension.

    I will definetly try that later and check if it helps. Thanks !
     
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