Back in the day (before third party hardware to do this) the only way to get Dreamcast online was to Dialup or the very rare and expensive broadband adapter.
Seeing as the Dreamcast shipped with a dial up modem, it was possble to set up your PC (also with a modem) and with a a couple of resistors, make a type of "dial up crossover cable". You would set up your PC to "answer" the call from the Dreamcast, then route the traffic to the PCs broadband connection. Zero phone line needed!
That's how it's done here as well. A USB modem attached to a raspberry pi (or just any pc) which answers the dial up connection from the Dreamcast, routing it through broadband.
I don't see how I would be able to since the broadband modem never came to NA so mine just has the dial-up one. Even if the tools linked here would work with dial-up, somehow: I don't have a landline to make the call. 😩
Edit: Needs a Raspberry Pi but doesn't need a landline. Interesting... Just need a raspberry pi and an RJ45 cable. $40-50 and I could play Chu Chu Rocket against other people again! 😃
I'm very surprised to see Alien Front Online in that list, for reasons. It was unceremoniously taken offline when the hardcoded IP of the bootstrap server was taken back by the telco when we switched providers.
I love projects like this and I have connected my Dreamcast, but never when I went to play has anyone actually been online. (anyone can check who's playing online with their Dreamcast here: https://dreamcast.online/now/ )