Dial backup is available in three varieties:
Floating static routes
Regardless of the method, you need to know what the primary and backup links are and what interfaces are involved. The type of interface as well as your overall routing design are influential factors as to which method may work best for you. For example, backup interface is not designed for running on a Frame Relay physical interface, but it works just fine if using subinterfaces. Always make sure the primary and backup links work individually without getting fancy. What I mean is, make sure both links work to begin with before you have one try to back up another.
Use the backup interface command to configure the ISDN link to be the backup ...