In the mid 90's, Sega had a vision of the future of gaming, On-Line gaming ! Unfortunately for Sega the world wasn't ready for an online gaming console (the first out of the box on line console was the Dreamcast) plus in Japan where the system was first launched, internet connections were still very expensive thanks to NTT. Not only did you have to pay for the internet connection but also a network "Game Credits" card that allowed you access to play online. As a result of the expenses involved the Saturn Modem never really caught on in Japan. In the US however it did have more success thanks to the free local telephone calls and the fact that you didn't need to buy credits. The US system was very simple. Basically if you and your friend had the same game and a modem you could just call each other directly and play. Because of this the US Sega Saturn modem still works to this very day as long as you have a dial up connection. The Japanese version on the other hand is long dead due to the servers that the system ran on being closed down.
A bit of pointless trivia for you. The Japanese Saturn modem was a sad 14kbps modem while the US one was 33kbps. Victor (JVC) also made a few prototype modems to sell alongside their V-Saturn however they pulled the plug on the idea probably after seeing how poorly Sega's modem was received.
As you can see from the picture below, there's not much to be found around the back of the modem. Just the serial number and a bit of text about licenses.
There are a few different packages for the Japanese Saturn modem so if you're after one keep an eye open for this set I have. It comes with the modem and phone cable like every other pack but this one also includes 3 software titles, Pad Nifty (Internet browser), Sega Saturn Internet Vol 1 (again, Internet browser) and Virtua Fighter Remix Network version. Other modem sets include either just Virtua Fighter Remix or Pad Nifty. There's also a DecAthlete set and possibly others that I've yet to come across.