One problem experienced by ISDN customers is that the router makes a connection even when none is needed. This is caused by the router interpreting any IP traffic as being destined for the Internet, and so creating a connection to transmit them.

A basic solution is to define the access list of the router, to define "interesting" routes that will be transmitted over the Internet. A sample solution is available from Cisco. Refer to their website for full information, but here is a brief summary:

access-list 100 deny ip 0.0.0.0 255.255.255.255 255.255.255.255 0.0.0.0
access-list 100 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255

This access list determines which IP packets will be interesting and activate the ISDN link. The access-list you should create depends on your particular network design.
Note: The command parameters for this example are:

access-list 100 deny ip 0.0.0.0 255.255.255.255 255.255.255.255 0.0.0.0
defines all broadcast packets as uninteresting.
access-list 100 permit ip 0.0.0.0 255.255.255.255 0.0.0.0255.255.255.255
defines all other IP packets as interesting.
dialer-list 1 list 100

This command points to access-list 100, which determines which IP packets will be interesting.