Well, Its always good to have a Firewall no matter what the connection. That way you can see whats coming in and going out. Do you have a 'terminal adapter', a router, or two shot gunned modems?I would suggest a router. A Netgear or a Cisco even. ISDN routers are available, but you dont hear alot about them due to the inefficient costs involved with ISDN. With the router and the firewall you should be able to take care of any attacking packet coming your way.