There are several issues with Fax over VoIP, due to VoIP being optimised for voice signals (smooth wave) rather than data signals (square wave), with the result that we cannot guarantee results.

Given that most documents are generated on computers, the recipient probably has a computer and email, and the limitations of faxes (low resolution, skewed documents); it may be better and easier to simply email the document.  PDF is also useful if you need to ensure that the recipient can print – but not alter – your document.

If you do need to send Faxes via VoIP, we recommend:

  • Connect the fax machine to the Vigor’s PHONE socket.
  • Use G.711 codec for sending faxes.  No compression will leave the signals closest to the original Fax (square wave) shape.
  • Enable the T.38 Fax function, if your VoIP device’s firmware supports it.  On the VoIP > SIP Related functions page, click the [Advanced] button. Then tick the box for “Enable T.38 Fax function”, and click the [OK] button to save the setting.

Please note that even with the above measures the quality may vary depending on factors which include:

  • whether the fax is going to another VoIP-connected Fax machine
  • whether your VoIP provider’s PSTN gateway supports T.38 Fax function
  • the quality of the PSTN phone circuit between the VoIP Providers PSTN gateway and the other party’s fax machine
  • the actual codec used for a particular call may be different from your preferred codec.