The Better Business Bureau calls it the "One-Ring Phone Scam": Scammers call your phone, let it ring once, and hang up. That prompts you—the scammers hope—to call back, allowing them to pile on charges while you're connected. The calls usually come from outside the US, but that's not clear to the victim because they're from countries that also use a +1 country code. Answer it, and suffer a $19.95 international call fee as well as a charge of up to $9 per minute, the BBB reports. (It shares area codes to look out for: 268, Antigua or Barbuda; 809, Dominican Republic; 876, Jamaica; 284, British Virgin Islands; and 473, Grenada.)
Many people won't fall for it, TechCrunch notes, so scammers have computers calling thousands of people every hour. That way, a few are bound to call back, thinking the unrecognized call might be an important one. If you get a suspicious call, TechCrunch suggests Googling the area code and number; that way you'll know where the call is originating from, and others may have already posted about it being a scam line. If you do get scammed, tell your carrier—they usually know about such tricks and will cancel the charges.