To get your Dreamcast set up, start by downloading this disc image.

Once you've got the image downloaded, either burn it to a CD via your preferred method, or copy it to your GDEmu SD card.

Boot up the disc, then with the phone line unplugged choose the "Connect to WebTV" option at the bottom of the menu, and select "Register". After a bit, the connection should error out. Choose "Phone Setup", then "Basic", and toggle the checkbox labeled "Tone dialing". Now plug your phone line back in and try connecting again. With any luck you should properly dial in and be taken to registration.