I have a brand new, unlisted number. In one day, I received four unsolicited phone calls without even giving my number out. This was one of them. I can only assume the phone company sold my number to them. I did not answer any of them. This one left a message - Jonathan from Home Security Center.
Since almost all numbers are recycled, there is a good probably that your "brand new" number is someone's "old number", and the number one reason for getting rid of the old number is due to unwanted calls.