You can use many different ways to do that. One of the most usable way is to configure the dialer watch. With dialer watch, the router monitors the existence of a specified route over the primary interface (serial interface in your case)and if that route is not present, it initates dialing of the backup link which is isdn.
Pl. visit following url for sample config and explanation
Yes, you will have problems with dialer watch. Dialer watch was not introduced until IOS 12.0 and Cisco recommends using at least IOS 12.1(7) because of bugs in early releases of dialer watch. FWIW I have been getting equivalent results for years using dial on demand routing and appropriately configured floating static routes. See the white paper on my web site on "BGP Driven DDR" for how to get dialer on demand to dial even if you can't guarantee the presence of "interesting traffic." More details are in chapter five of my book.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...