Depends on the call. Some are just trying to sell you a service of some type and hope that you sign up from the robot asking you to. Others are expecting you to answer and say something that they can use to claim that you signed up for their automatically billed offering. This was a common thing many years ago, people would call saying they were doing a survey and needed to know what type of printer / fax machine you had. They would ask you early on if you had time for a survey and when you said yes they used to that change out the question to would you like us to start sending you toner to which you now said "yes" so as far as their recording shows. So they would send you toner at a cost of 3x the normal cost. Some are trying to sell "legit" services but from fake numbers that way you have no way to know who it is until you answer. You then have the scams. Your computer is broke / has a virus so they need $400 in iTune cards to fix it. Or they want you to give them your credit card number so that they can give you a reduced rate. The auto hangup might be their system not realizing that you answered or if calls multiple people at once and one of the other numbers answered before you did.