What are you talking about? RJ45 is an eight pin networking cable. It has nothing to do with telephone.
You mentioned it yourself. Telephone uses six pin (RJ12 or RJ11) for upto three phone lines. If one phone line is all that is used. The center two pins (usually red and green) are all that is used. The other four pins may be ignored or taken out of the connector. But that doesn't mean a six pin wire and connector will not work.
I have a Brother MFC-8820D from the year 2004 with 1394 USB connector to USB 3.0 that has worked well. Comes with PaperPort but Microsoft XP.
I wanted to update, so I have a Brother MFC-L2750dw. A Wi-Fi connection to my router is available as well as a 1394 USB to USB 3.0.
Somehow I am knocking myself out of reaching the internet, due to my
telephone lines getting knocked out. They I have to reset my router.
Connecting the telephone line to the jack and then trying to FAX causes
a problem. I've looked at Vblogs on YouTube, and a few others have problems and suggest that it must be the need for a DSL telephone line that has only two pins.
I will have to use the old MFC until I straighten this out. I do not know why not use the USB 1394 to USB 3.0, since it keeps me out of the router. I only have one license for PaperPort, so I have to keep to use one computer.
Since that is the license, Wi-Fi used by other computers can't be used or interfere.