This seems to be one of a set of numbers being used to try to scam me.  I've received 10+ calls from fake number or "Unavailable" in the last 48 hours.  I've got to hand it to them--they're persistent!  But I never answer unknown calls unless the person leaves a message that I will want to return (and I say so on my outgoing message), but they never leave a message, so I have no idea how they plan to scam me.

Oh, I also add each number to a bogus contact, as I determine it's a fake, set it to not vibrate and give it a silent ringtone.  Saves me a lot of stress...

In my experience, they'll keep it up for maybe a week or so.  Then they'll finally give up when they realize that I truly DO NOT answer unknown calls.