Maybe it will work like this:

When you buy Mario Kart/Animal Crossing DS, you also get the DS Online Utility thing! Or it might come seperately.

You plug it in to a Phone Jack, and when the DS searches for other DS's to Chat with, it finds the DS Online utility thing instead.

Then, the Online DS Utility thing connects to the Internet, and you are then able to Chat with people over the Internet! ^_^

This is just what I think, though. As far as I know, none of the things I said in blue text are confirmed.