There are actual companies whose job it is to see what phone numbers are "live" or not. If you answer one of these calls, your number is automatically sold because now it is a "live" number. That is why when you answer they immediately hang up on you. They just want to know that someone is at your number and guess what--your number is sold to all kinds of people!! These people are making money when you answer your phone.
NEVER answer a phone number you don't know. Get "caller ID" if you don't have it so you can tell who is calling.