Another method is to call that number by some other number and tell him that you
are representative of certain company and want to know his reviews about that product. After taking his reviews, being very professional ask him about his name and address saying him that your company is having come gift hampers for him. I am sure that the person on the other hand will never refuse you to give his contact details to you. With the information take the help of police and treat that caller as you want to treat him. You will be no more in tension.