To use ISDN you must have a special ISDN telephone line supplied by your telephone company at additional cost. An ISDN "modem" is really a Terminal Adapter (TA). Like analog modems, there are internal ones on cards and external ones that connect to serial ports.
Configuring an external ISDN modem on a serial port is about the same as configuring an analog modem. The main difference is in the init string. Unfortunately, the init strings are different for different models of ISDN modems. There is often one AT command for a speed of 64k and another for 128k, etc. So you need to find out what init string to use and tell it to say wvdial, etc.
Support for some of these cards can be built into the 2.4 or 2.6 kernels or added as a module. The tty ports are ttyI2 for example. The kernel documentation has an "isdn" subdirectory which describes various drivers which support various isdn cards. A major website for ISDN is http://www.isdn4linux.de
For configuring, one might use the "isdn-config" GUI. A Debian package "isdnutils" is available. There is SuSE ISDN Howto (not a LDP Howto) which is translated from German http://sol.parkland.cc.il.us/sdb/en/html/isdn.html There is an isdn4linux package and a newsgroup: de.alt.comm.isdn4linux. Many of the postings are in German.