ISDN is different than plain old telephones. You may or may not have access to ISDN. ISDN equipment (on the Telco side) is a little different. . If they don't have the right stuff in their switches, it won't be available.
Call your telephone company to order ISDN. Rates vary widely depending on your location. The cost (per call) is the same as you would pay for a regular voice call to the same place (or modem call). You will probably wnat a straight BRI ("2B+D"). You can order each of the two "B" channels to be CSD ("Circuit Switched Data"), CSV ("Circuit Switched Voice") or CSVD ("Circuit Switched Voice/Data"). So if you need another phoneline, you can run that as part of the package on the same pair.
The equipment you need is some flavor of "Terminal Adapter" (a "TA"). THey can be as simple as a modem-looking thing (Like the USR I-Modem), or there are home/SOHO routers similar to the ones used by the cable/DSL folks. There are also internal TAs (like an internal modem). Prices are pretty broad...maybe ~$100-350.00 or more, depending on what you decide you need.
Good Luck
Scott