I've successfully done this using the DTR command in the cisco routers... We placed the ISDN line on each routers' serial interface 1. Upon failure of the 128k circuit or T-1, the Cisco router will switch traffic from serial interface 0 to serial interface 1 and automatically
establish an ISDN 128k connection. Additionally, upon restoral of the 128k circuit or T-1, the data traffic will revert back to serial interface 0 and drop the ISDN 128k connection. Basically, the router was initiating the DTR commands to the Adtran terminal adapter.