So, what is a one ring scam you ask? Simple. A company sets up a a toll charge site, often based in another country. They set up computers to robo-dial phones (particularly targeting cell phones with regular phone bills). As soon as the phone registers a ring, the dialer hangs up.
What happens next: Curiosity gets to a lot of people so they call back the "missed call." Since it is a toll charge site, you are automatically charged a fee - usually $19.95 (I am not sure why that amount) and of course, if it is international, you are also charged for whatever time you stay on the phone with international charges. Some international charges are not cheap either.
Simply calling the number back could cost you $30. How do you avoid it? Never call back a missed call, unless you know who/where it is from. Guess what? I just saved you $30. And to think I did it for free.
Here is a news story on the calls: