On Reddit, there was a post about a Crazy Taxi beta. After looking through the poster's website, I noticed something about a Sega tool for modifying and viewing the DC's internet settings. I was working on a program to view the contents of the DC's flash, and having that would help figure out a lot of currently undocumented parts stored in flash. The link was broken, but he was able to upload it again after I mentioned it.
Looking through what the program was displaying, I noticed that there was a setting for selecting between an external and internal modem, with UART settings listed underneath. (Screenshot here) It looks a lot like there is/was an option for having the DC use a external modem (presumably Hayes compatible) connected to the serial port, instead of the expansion port.
It seems unlikely that the option works. After all these years, no one has noticed this feature while reverse engineering different parts of the DC and its games? I think it's more likely just some leftover from before Sega finished developing the modem, and nothing really supports it. On the off chance anything does work, it's probably mostly untested and potentially buggy.
If it did work, it would be a huge improvement to the current DreamPi setup. You could just connect the serial port to a computer with a PPP emulator without needing any USB modem or dealing with line voltage. Both the Dreamcast and Raspberry Pi have 3.3v serial ports, so they could be directly connected. The host could skip dialing and just immediately say that it connected. A direct, digital connection between the DC and the host would have better latency than having to go between two modems. The program also lists higher baud rate options than possible over a phone line, up to 460800 baud.
Does anyone here know anything about this? I have a coder's cable, so I can try (at some point, maybe not right now) turning the option on to see if anything comes out of the serial port, if no one else has already done it.