This is a phone card.
Pre-paid cards are common in likely most countries, but Japan (more than any other) likes to issue special themed cards. Of course, there's a Sonic one. In fact, this is one of several that exist. These cards are useful, because even after you use up all the minutes of calling time, you can then add the card to your collection, much like a trading card!

This one is obviously for Sonic Adventure 1, as it has that iconic art of Sonic and the new logo. There is a phrase on it which reads: "I'll never look back, I've got no regrets 'cause time doesn't wait for me. I choose to go my own way". The card is shown here on it's retail/packing card and is from 1998.