There are 3 different ways for you to connect with hardware: DreamPi, VoIP, and Landline.

DreamPi is the most reliable method, but you need to have a few different things for it to work.

VoIP isn't as reliable as DreamPi, but all you need is a VoIP adapter.

Landline is the least reliable method, but if you have a landline already you don't have to buy anything.

DreamPi
Most Reliable - Hardest

VoIP
Moderately Reliable - Somewhat Difficult

Landline
Least Reliable - Easiest

If you're planning on connecting with a Dreamcast, follow one of the guides above to get your dial-up setup configured, then follow this guide:

Dreamcast
Decent alternative to a WebTV box for Dreamcast owners