Scam.
If you answer, there's a recording that goes "hello? hello?" then there's a momentary ruckus and then "oh I'm so sorry about that, I was having some trouble with my headset." - and then it launches into a script about how because you've booked your hotel with them before you've been selected to get a free vacation... We've all heard this story - it doesn't end well if you give them your card number...
How do these people sleep at night? Scum of the Earth.
When I get a call from unknown numbers, I press the speaker button and don't say a word. If no one says anything, I just wait for them to hang up. Sometimes I just leave the radio or tv volume on in the background. Waste my time, I waste yours...