You should use msn (multiple subscriber numbers) on the isdn-line. This is a standard feature on Euro ISDN lines. Often, an ISDN line comes with more than one number configured. This should be checked with the provider.
Then you can configure a unique msn number on each router. On IOS routers this is done with the isdn-answer command. This causes the router to only answer calls to the specified number and not any call (the default). The calling router must of course be modified to call the msn number.
I do not know the equivalent command on these ($#@!&*) 700 routers.
This is the way forward though.