2nd wan link only as backup

Hi,
We have a potential customer that wants to used the RED 2nd wan link only as failover.
in their case they have fiber or DSL is primary line and they want to use the 2nd link only when the primary fails, the reason is simple: the 2nd link is often UMTS or even ISDN and has traffic limits and high latency.

is there a way to configure this and is it a option to build this ?