Once upon a time, I knew someone who was a big fan of faxing on Windows. They used the Windows Fax and Scan feature of Windows 7 and when their Windows 7 machine died, they continued to use the same feature on Windows 10. Apparently it hadn't changed at all. I was not aware of this, but you can (or could) buy a modem with a legacy analog telephone jack on one end and a USB connection on the other. With it, this person was happily sending and receiving faxes.

All was well, until they replaced their router. Afterwards no more faxes. Debugging this was difficult: I was not at the location, I did not have remote access to the problematic computer, the fax enthusiast was a non-techie, I don't use Windows 10 much and I have never faxed to/from a Windows computer. This would have been one of life's little annoyances, but it became seriously important when the fax enthusiast came down with a medical condition that required faxing to/from doctors and labs. This was not something that could wait. Not even a day. Why the medical community still relies on faxing, I don't know - but they do.

What to do?

Simplify. Fewer moving parts makes it easier to narrow down the problem.

Since the problem started after installing a new router, both the router and the assorted wires and cables, that had to be uprooted during its installation, were suspects. Perhaps some wires weren't reconnected properly. In the end, the wires/cables were not the issue.

The Windows PC was running an ESET antivirus program, don't know which one exactly. One the one hand, it was objecting about a USB flash drive, which made me think that antivirus programs want to be active when new USB devices are plugged in so that they can scan the files for malware. Perhaps ESET had interfered with the USB modem connection? On the other hand, some searching online by the fax enthusiast found references to the Windows firewall causing grief for faxing and scanning. Again, Windows security software sometimes pokes around with firewall rules, so ESET was a double suspect.

As soon as the ESET antivirus software was removed, faxes could be sent.

But, the next day we learned that faxes could not be received. I suspected a firewall issue. When Windows connects to a new network, it asks the user if the network should be treated as public, home or corporate. Perhaps Windows considered the new router to be a new network? The question is typical of Microsoft and Windows. That is, its confusing and incomplete. Perhaps the non-techie fax user had not seen the message, or had responded wrong, somehow. Don't know.

It could also have been that the Windows firewall popped up its own warning during the first faxing with the new router and that warning was ignored or answered incorrectly.

Configuring the Windows firewall is an undertaking for masochists. Back in the days of XP it was simple and easy but with Windows 7 and 10, the user interface is a confusing, complicated disgrace. Rather than fight that battle, with a non-techie driving no less, we disabled the Windows firewall, and ... faxes can now be received.

The computer was not used in public, so the only thing the firewall was doing was protecting the computer from potentially bad things on the LAN.

Anyone who has dealt with a life threatening medical condition knows, there are things far worse than malware.