Hi RootbeaR,
Traditionally, external (hardware) modems have been the choice to avoid problems which were not Linux's problem but the lack of being able to work with Linux in a stable way due to lack of vendor support and a standard device interface. For example, I used to use a USR 5686E model when I had a dial-up 56k service. Very stable hardware modem. The problems I heard about were with internal Winmodems aka softmodem. See
Softmodem.
DSL usually comes with its own hardware modem, so no similar problems afaik.
-- Tom