I kind of think that call like this is a robo call hunting for active phone numbers. When you answer that tells the robo it is a real number with a real person. The robo then adds it to a list of good numbers to call and eventually sells that list to scammers. For scammers this saves them a lot of time by avoiding dialing non-working numbers. At least that's what I would do if I was in the scamming business!