DTMF authentication for ISDN router

1. SnapGear firewall, ADSL router and backup ISDN router

Hi to everyone,

I have the following configuration:

LAN ----> SnapGear Firewall ----> ADSL Router ----> Internet

I would like to re-use an ISDN router to setup a failover connection to the
Internet, like this:

LAN ----> SnapGear Firewall ----> ADSL Router ----> Internet
                                               ----> ISDN Router  ---->

I belive I've the following options:
1. static routes with different metrics and dead gateway detection: this
only  work if the firewall - ADSL router connection goes down, does not work
if the ADSL router - Internet connection (the one I'm concerned about) fail.
2. dynamic routing protocols, (ie gated or zebra) which should work but are
not installed on SnapGear...

It seems to me it can't be done with this hardware...
Maybe someone has a brighter idea ??? :-)

Kind regards,
    Corrado

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