Goldstein and associates threatening call to my employer Franklin Louisiana

Received disturbing call from (517) 489-2259 Private Investigator who wouldn't give me full name, staying that he is attempting to serve me with some papers for mediation hearing if I don't call Goldstein and Associates at 877-527-9594 (during another call that number was 888-323-7874) they would call my family friends and work place. I called back the number provided. Man on the phone stated they purchased my account from cashcall and I owned them 7,000. I asked politely to provide me written notice and all documentation on this claim for my review. Gentleman on the phone began to screaming on the phone and did not even let me ask questions. I don't even know the address of this company. "Private investigator" keeps calling numerous times a day not only my number but my friends numbers. I feel so harassed that I cannot sleep at night. Previously, I had one collection agency that called me and sent documents and explained what I have forgotten to pay. They also provided me 30 days to review all information provided.  

Goldstein and Associates has been harassing me and my friends for the last three days and when I called them again - on another- third phone number they provided they failed again to agree to sending me written notice. I feel helpless and don't even know who these people are. I am begging for some help here.