There was an article in 'Communications Week' last week about an ISDN
device that would hook to a standard serial port.  Looked like a modem
to the machine, but a digital modem.  Only used one channel (56/64), had
a phone jack in the back so you could use a normal phone (or modem ;)
at the same time.  3com Impact ISDN $599 list

1-800-NET3COM