Troubleshooting: Fax

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At times an install will have issues making or receiving faxes. Typically a fax machine is integrated via an FXS gateway. If possible, it is best to run analog fax lines direct to the fax machine, skipping the PBX entirely, especially if faxing is a big part of the users business. Here are some things to check to ensure all configuration is done correctly on our end.

  • Does the issue happen on single page faxes, or only multi-page faxes? Do inbound faxes fail, outbound faxes fail, or both fail?
  • Ensure the FXS Gateway which integrates the Fax machine is configured to use a Fixed Jitter Buffer. Since duration of signal and pause are so important when faxing, using an adaptive jitter buffer can cause issues with faxing, especially multipage faxes.
    • Try connecting a standard analog phone to the gateway and make calls, if no problems are experienced its a good indication that there is nothing inherently wrong with the gateway itself.
  • Are you faxing over a PRI? Its advisable to ensure the provider knows that your DIDs will be used for faxing so that they can configure things correctly on their end; especially with an Integrated Access PRI
  • Ensure the Fax Machine is set to a maximum baud rate of between 9600 and 14400. It has been our experience that the faster rate fax machines are more prone to issues when VOIP is involved.
  • To disable Voicemail on your fax extension, enable Busy, NoAnswer, and Unavailable forwarding. Set them to go to Destination Services=>Congestion.
  • Are you faxing over SIP trunks? This is not typically advised as any jitter or packetloss on the path across the internet can cause the fax to fail. You can investigate via the Traceroute feature in the PBX, found under Diagnostic=>Network. IPitomy does not support fax over a SIP trunk as it is far too unreliable. Fax is an analog modem-based technology that is reliant on direct handshaking and transmissions, and any lost data at all can cause the fax to fail.
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