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I REMd out the USBCOM line and Mode returned the same information. Therefore it is the Serial ports that ArcaOS recognizes.
Opening PMFax now only displays for Ports: FMD!; COM1; and COM2. Since then the USBCOM line might have been confusing it, I unplugged the USB cable from the modem. However, since I still can't receive any calls, more investigation is required.
(The modem manual cautions not to have any line splitter or other devices between the wall jack and the modem. However, I'm aware of a situation - parallel to your experience - where a hospital didn't have any protection between its telephone line and its computer and a lightning strike fried the latter. Therefore, I'm still going to keep the line going into and out of my Tripp Lite UPS.)