What you can do is configure hsrp have the router with the leased line act as the active router and also put your track statement on the leased line . This way when the leased line goes down the backup will become the active router and the other isdn line will kick in.
You should track the leased line under the HSRP process on the active router (the one with the leased line). You should also have some routing protocol running between the routers on the lan that connects them, configured in a way that the backup router prefers the path through the active router instead of using its own isdn interface. The backup router should have a floating static (default) route point to its isdn interface. Hopefully, when the main WAN (leased line) link goes down the active router will decrease its priority and the standby takes over. At the same time the standby router should loose its preferred path through the former active router and its isdn kicks in due to the floating static. I almost forgot one thing... The active router may also need a default route with higher admin distance pointing to the standby router, just in case there are some hosts that are using the real ip from the active router instead of the HSRP address.
Hope this helps.
P.S.: if your leased line is using Frame-Relay you have extra work to do...
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