Have a look on section dod on how to configure dial on demand - and on the dangers attached to it. Also you may have a look at section remote when you try to connect to a special remote ISDN device. The new ISDN drivers in 2.0.36 defaults to manual dialmode, not autodial. Use this if you want to be sure that no connections will be made. Some daemons or other pc's that use this interface can cause this.
Incoming connections are also possible.
In this mode, the interface will never make any connections on its own. You have several choices to selectively allow users to do ISDN transactions. It has been reported that you also may have to change group and permissions on the programs ipppd and isdnctrl to 'isdn'. Then all users not in the group 'isdn' have no reading or writing privileges for the ISDN ttys. Those allowed to use ISDN have to be explicitly added to the group 'isdn'.