I hat Problems with dod with isdn after loosing the connection.
Reason was the delete default gateway.
In the script network-funktions in function add_default_route
 is the line

/sbin/route add default gw ${GATEWAY} ${GATEWAYDEV}

in My opion this is wrong and cause the problem.
My solution is to replace the line with this:

/sbin/route add -net ${GATEWAY} dev ${GATEWAYDEV}

May be this is not the best solution but it work.

By christian