The built-in fax works great, it stores incoming faxes in /var/spool/asterisk/fax/
The only change I need is for it to store the faxes in /var/spool/asterisk/fax/EXTENSION_NUMBER/
I have a couple of extensions, and that would help to keep the faxes separate from each other.
I assume that I need to modify the fax dial plan, but I’m not sure where to find it.