Interchex - Harrassment

Hello,

I have been receiving calls from Interchex on my cell phone, work phone number (numerous times a day) and now to my family members. They are saying they are calling from Seminole County Courthouse (the county that I live in) and that they are investigating me for bank fraud. They even tell the people they reach this. My sister called me freaking out that they said they were trying to find me to arrest me for bank fraud. A few weeks back I did take the call and tried to find out what they wanted. They said it was from a payday loan in 2008 and had my social, date of birth, old bank info etc. I told them to send me proof of the debt and then I would speak to them. There was a time I had some payday loans but have settled this was years ago. I never had one for more then $300 and they are saying I owe them $600. The verification they sent me was on my email and it was on a Interchex letterhead but shows nothing of the original loan company amount signed agreement nothing.. just them writing what I owe. How can I stop this?? I am so afraid the next call to my work they will get my manager and tell them also that they are trying to arrest me for bank fraud. I would be fired. Help!!!!!!!! If I knew this was legit and there was proof of original loan I would speak to them.