Taken partly from Dan Kegel's ISDN Page:
ISDN cards are like modems, but 5 times faster. They require special telephone lines, which cost a little (or a lot, depending on your phone company) more than normal phone lines. They provide up to 64 kBit communication speed on one channel and multiple channels can be combined.
This category contains applications that display or configure ISDN configuration and provide a KDE GUI.
More information