@Stew: Yes, as long as the line is "live," then you can use any regular "landline" telephone, modem, or fax machine on a particular telephone line/socket. Therefore you should be able to connect your Dreamcast with no router required. (Technically speaking, the ISP would have a router located at their building, but you don't need to worry about it.
@Shin: I believe you could take the route of Dreamcast to DreamPi to Wi-Fi to mobile Wi-Fi wireless router hotspot (such as a MiFi). Of course, you don't need to use a dedicated device like a MiFi; you can just use a typical Android-based smartphone and create a "tethered connection."
For more information about that, see
[ https://lifehacker.com/5447347/how-to-t ... roid-phone
Note that you can use either your Wi-Fi or mobile network connection; if the case of the former, you are good to go without extra permission; with a mobile (cell) connection, your mobile carrier must support tethering, so consult their documentation or support personnel.