[plug] ADSL with modem backup

[Luke Dudney]

I'm trying to build some fault tolerence into a remote ADSL connected 
RedHat 7.3 machine.

Has anyone here successfully implemented an ADSL connection with a PSTN 
modem dial-out as a backup?
What I'm looking for is some sort of method to detect an ADSL outage 
(for a set period of time - say 30 minutes) and then automatically dial 
the modem.
I want to continue testing the ADSL connection, and if it succeeds then 
drop the modem line, reset default route etc.

I'm using the standard setup scripts with RP-PPPOE to dial the adsl 
connection.
Are there any scripts available on the web that either achieve this or 
demonstrate in some other way how it could be achieved?
Or does anyone have any other thoughts?

Cheers
Luke.