Hello?... Hello?

Can someone please explain why I get phone messages where a person says "Hello?... Hello?" and then hangs up. It sounds like they're in a room of people saying "Hello?...Hello?".

What the hell product or service are they selling, and do people really star69 to call back? The Nigeria e-mail scam works, so I guess getting lonely people to return a weird call from an unknown voice has to be a piece of cake.