The Tagged Image File Format is not very well known, but it is actually more common than you might realize, and since it is natively supported on Windows, MacOS, and Linux, TIFF files can be viewed on pretty much any computer with the most basic graphics viewer. A subset of the TIFF file format has for a long time been the de facto file format used for faxes.

Since Asterisk will receive and store faxes in TIFF format, there is no post-processing required. Once the incoming fax call has been completed, the resulting TIFF file can be opened directly from the folder where it was stored (or perhaps emailed to the intended user).

Once Asterisk has received a fax, the resulting TIFF file needs a way to get to its final destination: a person.

The key consideration is that unless the information that Asterisk knows about the fax is sufficiently detailed, it may not be possible to deduce the intended recipient without having someone actually read the fax (it is common for a fax to have a cover page with the recipient’s information written on it, which even the most capable text recognition software would have a difficult time making sense of). In other words, unless you dedicate a DID to each user who might receive a fax, Asterisk isn’t going to be able to do much more than send all faxes to a single email address. You could code something in the dialplan to handle this, though, or have an external cron job or other daemon handle distributing the received faxes.

A simple dialplan to handle fax to email might look something like this (you will need the mail program mutt installed on your system):

exten => fax,1,Verbose(3,Incoming fax)
; folder where your incoming faxes will initially be stored
  same => n,Set(FAXDEST=/tmp) 

; put a timestamp on this call so the resulting file is unique
  same => n,Set(tempfax=${STRFTIME(,,%C%y%m%d%H%M)}) 
  same => n,ReceiveFax(${FAXDEST}/${tempfax}.tif)
  same => n,Verbose(3,- Fax receipt completed with status: ${FAXSTATUS})

; *** This line should not have any line breaks
  same => n,System(echo | mutt -a ${FAXDEST}/${tempfax} 
-s "received fax" somebody@shifteight.org)

Obviously, this sample would not be suitable for production (for example, it does not handle fax failure); however, it would be enough to start prototyping a more fully featured incoming fax handler.

You may have a dedicated phone number for receiving faxes. However, with Asterisk, that is not a requirement. Asterisk has the ability to detect that an incoming call is a fax and can handle it differently in the dialplan. Fax detection is available for both DAHDI and SIP channels. To enable it for DAHDI, set the faxdetect option in /etc/asterisk/chan_dahdi.conf. In most cases, you should set this option to incoming. Table 19.1, “Possible values for the faxdetect option in chan_dahdi.conf” lists the possible values for the faxdetect option in chan_dahdi.conf.