USB 56k modems under linux

My dad is going to be flying out in a few weeks and he has never owned a
computer (much less used one extensively), but he does have an xbox...
The problem being that the xbox has jack for ports. Basicly USB and a nic.

Does anyone have any experience getting usb 56k modems to work under linux?

OR... Is there a 56k beast that is connected over cat5?

Thanks for any possible help.