Fax over VoIP

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Fax over VoIP is not reliable and does not work most of the time.

It is possible to get a nearly 100% rate in rare circumstances but there is a very very low chance of this.

The broad explanation lies in a technology mismatch between FAX and VoIP, as explained in Steve Underwood's article here.

Steve Underwood developed the SpanDSP package, which helped to implement FAX support into Asterisk, Freeswitch and other open-source VoIP platforms.

The T38 protocol is supposed to work reliably over VoIP, but the problems with this protocol are the following:

  1. Different hardware vendors do not follow standards and T38 implementation on various hardware is mostly broken.
  2. Asterisk does not support T38 very well.

See also