ISDN or CABELmodem and Linux

Hi all, yes I know there is little chance to find an internal ISDN card that will be supported by any flavour of Linux.
But will be there a Version of USB support in the Linux space?
Or frankly is there a ISDN or Cablemodem
supported by the seriell Interface?
Why, well here in Europe the speed of any modem is never …..and the Telco’s are far from a flat-rate.