Like telephone companies, Internet providers charge either for a dedicated line (usually with unlimited use) or for connection time. You should estimate how much time you expect to be using your ISDN connection. Remember that the overhead of reconnections is low, so reconnecting each time you want to log on to the Internet is often an acceptable approach.
If you plan to connect a network of computers by using an ISDN bridge or router, find out if the provider can assign you a subnet of IP addresses or route traffic to a network of addresses that you already have assigned. There is no point of going to the expense of purchasing an ISDN router if you can't connect more than one host to the Internet provider.
Also, ask the ISDN provider for recommendations for hardware that's compatible with its equipment. This will ensure maximum performance and the fewest configuration problems.