Cannot fax from Ubuntu through HP Envy Photo 7800 All-in-One on the local network

[Md Ayquassar]

I use Ubuntu 20.10 with hplip 3.20.5+dfsg0-3build1. The AiO fax is physically attached to the router via the fax cable and via the Ethernet cable, and my Ubuntu machine communicates with the router through WiFi. Printing from Ubuntu works, faxing directly with the AiO machine also works, but NOT faxing from Ubuntu. I tried several ways of sending a two-page PDF through the fax, in particular several ways of choosing the fax driver and the device URI, but, never mind what I did, the fax did not get sent. For example, the recent attempt put

[14447.505158] audit: type=1400 audit(1605916929.875:51): apparmor="DENIED" operation="open" profile="/usr/lib/cups/backend/cups-pdf" name="/etc/cups/" pid=11678 comm="cups-pdf" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

into the syslog. Is it a bug? Very likely. Does it have anything to do with faxing? Perhaps, no idea. Some of my faxing attempts ended with the "abgebrochen" status, others with "Die Druckerkonfiguration ist nicht korrekt oder der Drucker existiert nicht mehr." status, yet others with "Device communication error" accompanied by Code 5012 and "Please correct the problem and try again". Yet another attempt ended with "FileNotFoundError: [Errno 2] No such file or directory: \'/home/my_username/.hplip/hp_fax-pipe-146\'" deep in /var/log/cups/error_log .
Starting hp-toolbox prints

error: Unable to communicate with device (code=12): hpfax:/net/ENVY_Photo_7800_series?ip=192.168.1.225
error: Device not found

to the console in red, although the AiO device is on.

All of these are probably bugs of various kinds which probably have something to do with faxing, directly or indirectly.

I guess, my main point of criticism is that besides 10 to 20 bugs such as above, it's nowhere clearly written, in a step-by-step way, how to send a fax, what to expect at each step, and what or how to troubleshoot if the expected thing does not happen. <humor>Or, at least, whom to blame for what. Blaming someone always helps, at least mentally.</humor> For example, https://developers.hp.com/hp-linux-imaging-and-printing/howtos/faxing mentions the "Items" tab, but on my machine, there is no such tab in hp-sendfax or hp-toolbox. The description on https://wiki.ubuntuusers.de/HPLIP/#Faxen is equivalent to empty. Yes, I did run hp-sendfax -pENVY_Photo_7800_fax -fTargetFaxNumber
(where the TargetFaxNumber is 03212... in my country).

In short, how on earth do I fax my PDFs from Ubuntu through HP Envy Photo 7800 All-in-One?