I have a T1 for wan connectivity back to our main office from a branch that is backed up by an ISDN connection to our main office, I also have a DR site in another state that has the same config as our main ISDN router at corporate. What would be the most efficient way of telling the branch router to dial that out of state router if the home ISDN router is unavailable? Would secondary dialer string suffice? Secondary dialer?..
There are always possibilities of details in a local environment that would change the advice that we give. But given your description of your requirements I would believe that a second dialer string would be sufficient to have the router dial the DR site if the main ISDN router were not available.