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Fax machine can receive but not send

Discussion in '3CX Phone System - General' started by netelligence, Sep 19, 2014.

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  1. netelligence

    Sep 24, 2012
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    I'm sure no one likes the dreaded fax machine and I usually do my best to show clients how they don't need one.... however, there is this one client............

    So I have a patton m-ata and have used the instructions provided by 3cx to configure it

    The instructions call for setting the fax machine as an extension. I did that and faxing did not work. I set the fax machine up as a "fax extension" in the "fax machines" section and the fax machine started receiving faxes perfectly. Happy day. So for good measure I tried sending a fax from the machine and that doesn't work. You can here it call out and make the connection and then basically time out.

    I have tried 2 different voip providers and verified that they each support T38. I feel like I'm close, but I'm stuck on what to do at this point.

    Any help would be greatly appreciated.

  2. lneblett

    lneblett Well-Known Member

    Sep 7, 2010
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    I think the issue is that by using the fax extension, you are now using the fax server feature which does not support sending as you might think. The media server for the port supports T38 on the inbound, but not for outbound. As a result, the outbound is being handled as a standard voice call.

    This is why the instructions call for the device to be used as a regular extension. This way the ATA can negotiate the proper codec. I have tried a number of different ATAs to accommodate faxing and I met with some success, but the success ratio went down in proportion to the length of the fax.

    There is a HTTP device that allows for faxing across the internet with a high degree of success. It is independent of the phone system but there is the cost of the appliance and then the monthly service. The service is comprised of a website that allows for the viewing of all sent and received faxes as well as being able to download, re-fax or even email from the portal directly. It does, of course, work as a conventional fax machine as well.

    If you like, you can PM me and I can provide more details.
  3. eagle2

    eagle2 Well-Known Member

    Apr 27, 2011
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    If you are using a T.38 capable ATA like Patton M-ATA and you have configured fax extension in 3CX and put its credentials into ATA and you can receive faxes on a machine attached to the ATA, you should be able to send faxes as well.

    I can confirm 3CX is completely transparent for T.38 in this setup and the ATA is negotiating T.38 transmission with the provider. I have successful experience with both Patton M-ATA and AudioCodes Mediant MP112/114/118 being most successful devices on the market. Success rate with Cisco SPA series or Grandstream is little. The result is exactly the same if ATA is connected directly to SIP trunk without using 3CX PhoneSystem at all, so 3CX is irrelevant to the problem.

    Having analyzed VoIP traffic with wireshark, most ATAs are not capable to negotiate successfully T.38 session with the provider, when sending faxes, and in most cases negotiate a G.711 instead, which logically is little successful. Playing with receive/transmit gain can improve the result, also timeout while negotiating T.38 is important (not configurable in most ATAs). Usually adapter gives up too early and reverts to G.711 (usually in less than 6 seconds, which in most cases is not enough for long-distance or international calls). Of course the result is dependent on provider as well, meaning whether implementation complies fully to T.38 protocol.

    I would advise to ask Patton for support, you may collect wireshark captures, and if you like, without using 3CX PhoneSystem in between. Having stabilized the behavior of M-ATA, you may reconnect it to 3CX PhoneSystem.

    Good luck.
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