External modem

Hi, AFAIK, installing a PCI modem causes problems in Linux, but external modems should always work. So I purchased a Sitecom 56k external modem. But alas, it was not recognized by Yast1 or Yast2, although I selected the right serial port (/dev/ttyS1). Has anyone got a Sitecom external modem working? Or is it true that not all external modems work? TIA, SH