I'd like to append a few comments to add to previous replies here:
 
* The first response (that I saw) simply stated if you had the CSU/DSU-like
  interface, with a common user-side interface (RS-232) you had it.  Perhaps,
  but speed-wise you can't do more than 38.4 kbits (that I know of) into a
  DTC port (and only the newer DTCs will do 38.4k).
 
* Chris Bartram's response was a more workable solution, but bear in mind
  you must have the remaining network infrastructure on both ends to handle
  (telnet and/or NS/VT connections) and hpterm emulation.
 
ISDN is just a protocol, most commonly used for medium-speed serial links.
You can communicate between routers or between multi-protocol repeater/bridge
solutions.
 
Jeff Kell <[log in to unmask]>