Megadrive, you don't need an ISP or anything for Net Link. The way it works is that one modem simply makes a direct phone call to the other. There are no servers, and there is no internet to go through... the two systems are just programmed to know how to interact through a standard phone call, and they allow the gamers to play and also to talk in a lobby before and after the match. That's the X-Band direct-dial technology.
The catch is of course that long-distance communication is a long-distance phone call. My long-distance is a flat rate, so I wouldn't be paying any more to call you.
Since Net Link is as direct as making a phone call, the speeds over a dial-up line are actually as fast as broadband - 20 to 30 ms pings! Unfortunately, the Net Link games were among the early attempts at making networked titles, so the developers didn't understand how to hide lag well. They actually programmed the games to try to be almost as precise as the offline versions, meaning the software waits for response before updating on a frequent basis. This keeps player tracking accurate, but gameplay a little lagged. And depending on how far away the two players live, the lag can be noticeably higher.