This is a particularly clever scam that doesn’t even need any human interaction.  Here’s how it works.  Your phone rings once, then hangs up.  When you check your last call, it gives a number you don’t recognise, so you call it.  At this point you’ve just connected to a premium rate number and being charged through the nose as you put up with some godawful muzak while you wait to speak to someone or listen to a prerecorded message.  Every second that you’re on the line, the scammers are making money from you, and you won’t know about it until your next bill.  All the scammers need is an autodialer and a premium rate number.