If your house has phone jacks but you don't use your land line for anything anymore, you could essentially hook one end of the phone line simulator up to where the phone company's line would come into your house and the other end up to a computer or router like is shown here that is set up to share your broadband. At that point you could plug your DC into any phone jack in the house and make the call to your router... your own personal, in-house, dial-up ISP.
That would be a pretty sick setup! Then you would be able to connect Dreamcast to any phone jack in the house. This will have the same effect as an RJ-11 splitter. Just remember though that on the other end it would still be connected to only 1 line on the Netopia device so you wouldn't be able to connect multiple Dreamcasts simultaneously unless you have two or more separate phone lines wired through your house.
Also on the note of phone line simulators; the Netopia supports 3 different ring patterns which are indicated on page 79 of the user manual. So it shouldn't be an issue if your phone line simulator is from a region other than North America.
Here is a photo of my Teltone TLS3 phone line simulator: