> Is there a way to prevent an idle timeout from an isp.

As others have suggested, ping is probably enough.

If ping fails, a HTTP request every few minutes should certainly do
the trick.  I use a URL with a valid host (i.e. not the host below)
and a non-existent file to minimise data traffic.

echo $$ > /var/run/bigpond-keepalive.pid
while :
do
    ftp -o /dev/null http://www.example.com/keepalive > /dev/null 2>&1
    sleep 300
done

Arrange that your ppp-up script call the above and your ppp-down
script kill it, and you're done.

Cheers,

Giles