-  Repetitive nuisance calls. Constant call center background chatter.
 -  Suspect - information data mining calls.
 -  Although they fully expect you to provide them with information, they themselves refuse to identify themselves or the company -they- are representing.
 -  Too, if you can get them to stay on the phone long enough, it becomes very apparent that they are unfamiliar with terms like:  "Not listed in my directory", and "You have the wrong number"

the caller will not stop calling and asks for personal phone number.  very Shady