I'm getting several robocalls (automated calls) from Harris & Harris Debt Collectors saying that I owe a debt but they won't tell me any more information. I don't know how to stop them. I found that this is happening to many other people from this post http://badfinance.org/profile/823/harris-harris-ltd and http://badfinance.org/review/1924253/harris-harris-ltd-debt-collection
You should check your credit report. Sometimes debt collection shows up on your credit report. If you do this, you will hopefully be able to see what is causing them to call you. If you find that the debt has already been paid, you can send them proof and then send documentation to all of the credit bureaus as well to dispute the claim.
Can you record the calls? You might need them as evidence in the future. If you sue them for harassment or anything like that.
There are a lot of websites where you can check your credit report free of charge. One of the best options is annualcreditreport.com
You should look through your credit report and make a list of all the debts it mentions. And look in your records to find out if all those debts are valid. If any are old, you should find out the statute of limitations for collecting them in court.