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Don't plug a phone line into the USB port, period.
Try plugging the phone line cord into the jack on the printer that is labeled 'LINE' or 'L', and NOT the one that is labeled 'PHONE' or has a phone handset logo next to it.
How does that work?
You don't fax over USB.
So yes, USB and phone line both.
Does it work now? before I leave off from this case, I'd like to know.