Hi
This may not be the solution, but just a suggestion.
I suppose you want internet access via the ISDN. Since the PIX needs a default gateway for 0.0.0.0 traffic you should talk to your ISP and get some public IP adresses for your routers and PIX outside interface..... 2 IP addresses should be enough (255.255.255.252 subnet mask).
You can't set a default gw if you get a random IP address from a DHCP server each time the router dials.
Best regards
Jan Beining