i would check your credit report to see if they are listed as a collection. you can check it for free at annual credit report.com..
If they are there, send them a debt validation letter, which requires them to tell you about the debt. meaning original creditor, amount, account number ect..
if they still call you,simply send them a cease & desist letter which should say they are not permitted to call your number or contact you by any means except mail. also, important to mention not to call at work either...BUT keep copies of everything you send them and send it certified return reciept mail..you never know when it will be important to have copies..