Well you can't just connect your console to a modem and have it work. IDSN/DSL modems will not work. If you mean an external fax modem, you need to create a modem to broadband bridge using a linux virtual machine. Get real dialup and plug your console into a phone jack. Or setup one of these
http://blog.kazade.co.uk/p/dreampi.htmlTo answer your question though, the settings are typically this:
Username: (username provided by dialup ISP, or set in virtual machine like i mentioned
Password: (Dialup ISP password, or set in linux vm)
Phone number: (Dialup ISP phonen number, or number specified in linux)
Since you mentioned you do have a phone line, try looking into free dialup ISPs in your area. NetZero and Juno will not work in case you come across those. If there is no free ISPs, you could pay for a full year for around $70. The price of a DreamPi setup. Or monthly. But you can't plug your console into an IDSN router and dial your home phone number hoping your router will negotiate the call
If you get an ISP, make sure you don't need to run any of their software. Because the Dreamcast can not run it obviously. That is why Juno and NetZero will not work
Hope this was of help