What do you call a UART on a USB port? These things are MCUs with a USB on one side and a UART on the hardware side. The software on the PC makes them look like a UART to the PC, but the PC is really talking over USB.
The question is how do you know that? As I have said, I know how to do this stuff under Windows, but not under Linux.
I've also posted that when I do an ls /dev/tty* I get a list of tty, tty0 through tty63, ttyAMA0 and ttyprintk. This list does not change when I plug the device in or unplug it. I also spent some time plugging in a number of USB UARTs and none of them showed up any differently this way. I did find one that crashes the rPi as soon as I plug it in. I expect this is part of the power issues the rPi has.
What do you mean, "announces itself"? Where do you see that?
I'm not totally clear on the difference. A modem is connected to the PC via a UART. To talk to a modem the software talks through the UART. I suppose there is the AT command protocol that is used to control the modem which has nothing to do with the UART. I guess the modem is like another layer on top of the UART although that assumes a smart modem as opposed to a dumb modem which just transmits the data it sees one bit at a time.
Thanks for the effort, but no, this has not helped. I understand modems and UARTs probably more than most people. I don't know much about Linux and I don't see anything here that explains that. I'm a complete noob at Linux, not computers.