Rosenthal, Stein, and Associates - harrassing me

Hi - I got into some PDL loan trouble a few years back, I worked my way out and paid off my 2 debts via a collection agency. For the last three weeks I keep getting calls from a collection agency going by Rosenthal, Stein, and Associates. They are very suspicious, telling me I owe $1000 or I will be sued the next morning. They refuse to tell me who the original creditor is and get very hostile when I tell them I need more information, when I ask to whom I speaking with they scream "Figure it out" and then hang up...it's very disturbing and will not end...im talking at least once a day...yelling into the phone yet refusing to send me anythng in writing. I looked on the internet and they are a legitimate business. I don't believe for a second they are going to sue me, but how do I make this scary harrasment stop?