Set up Fax over IP (FoIP)

Fax over IP (FoIP) is the process of using T.38 protocol to send a fax from a fax machine to another fax machine over the Internet. This topic describes how to enable T.38 for extension and trunk respectively, and how to change T.38 settings to improve the Fax transmission over VoIP network.

Enable T.38 protocol for SIP extension

If you want to register a SIP extension on a SIP compatible fax machine, you need to enable T.38 Support for this extension.

  1. Log in to PBX web portal, go to Extension and Trunk > Extension, edit the desired extension.
  2. Go to Advanced > VoIP Settings.
  3. Select the checkbox of T.38 Support.
  4. Click Save and Apply.

Enable T.38 protocol for SIP trunk

If you want to use a SIP trunk to send or receive faxes, you need to enable T.38 Support for this trunk.

  1. Log in to PBX web portal, go to Extension and Trunk > Trunk, edit the desired trunk.
  2. Go to Advanced > VoIP Settings.
  3. Select the checkbox of T.38 Support.
  4. Click Save and Apply.

Change T.38 settings

If the Fax over IP doesn't work, you can change the T.38 settings.

  1. Log in to PBX web portal, go to PBX Settings > SIP Settings > T.38.
  2. Change the T.38 settings.
    • T.38 Max BitRate: Set the maximum bit rate of the fax transmission.
    • No T.38 Attributes in re-INVITE SDP: If enabled, SDP re-invite packet does not contain T.38 attributes.
    • Error Correction Mode: If enabled, after receiving the packet for a complete fax page, PBX notifies the transmitting fax machine of the frames with errors. The transmitting fax machine then retransmits the specified frames.

      This process is repeated until all frames are received without errors.

  3. Click Save and Apply.