Networking with 2 ISDN lines

A Client has two different locations with
ISDN lines installed at both locations. They
wish to connect the two locations with routers and the ISDN lines. (ex. at one loc.
they enter an appointment date. This set
appointment date should be transferred over
to the other loc. using ISDN and router)
My question is this. Is there a way to get
the routers to talk to one another and if
so how do u do it ?