Before you read any further, if you want to try this, you need a free license from Digium for a single port of FAX - the FFA - Free Fax for Asterisk - it's free, but it took them three days to get the license to me, so do this first!
I don't know, maybe I am dense, but I have been scouring the web for DAYS trying to get this working because I NEED to get to Asterisk 1.6 for the better CDR's, SIP-over-TCP and all the other goodness that 1.6 has to offer - but I also NEED to have NVFax Detection working because our customers demand it. I have read everything I can find, to no avail, so I finally started hacking - if someone else has done this and gotten it working, Google and Bing sure can't find it.
First the Obvious:
You will need the updated source for app_nv_faxdetect.c and app_nv_backgrounddetect.c - get them here if you don't already have them:
Once you have the files, they will need to go into the apps folder of the Asterisk source files before you compile them.
Now, the Non-Obvious Stuff (Again, maybe it's me...)
1) Remove app_fax.c from the apps folder in the Asterisk Source - this conflicts with res_fax and res_fax_digium that you need to download from Digium. This is probably the most counter-intuitive part of the install to me, and after reading over the docs several times, it's just not there to remove it - if you don't, Asterisk crashes when you try and load it. If it's already compiled, just remove the file from the /usr/lib/asterisk/modules folder - app_fax.so
2) You will need to compile spandsp-0.0.6pre12.tgz for the NVFax to use - it is also on my ftp server if you need it.
3) Get the FAX files from Digium - or get the files from me if you want the quick route. The have to go into /usr/lib/asterisk/modules also.
Now, for the last step, a comment, and then a question - you need to edit /etc/extensions.conf and search for 'rxfax' and replace it with 'ReceiveFAX' - case sensitive - this is the only Dialplan Change I made to be able to receive FAXes - in fact, while I have been typing this up and posting the files to my FTP server, I have received an 86 page FAX and a 116 Page FAX - I used my Wife's laptop and faxed myself two of the books she is working on - they went without a hitch. I also sent several 3-5 page faxes from web pages that were HEAVY with graphics and they worked fine also.
So here's the question - where is the template for extensions.conf so that as I update my FreePBX, I won't have to keep going back into that file and fixing it?
P.S. - I am working on a procedure to install a scratch-load CentOS 5.4/Asterisk 1.6/FreePBX 2.6 machine - including the instructions specific to the FAXing - I will post that also when it is complete.