NCO Financial Systems NEED HELP TO GET THESE PEOPLE TO BACK OFF Houston Texas

I need some serious help with NCO Financial Systems. We are in the military and unfortunately have had 3 companies over the last 3 years (2 are commercial and 1 military) that have had employees that have given out (sold) our information (name, ssn, address, phone #, etc). Here lately we have been receiving phone calls from NCO in regard to collecting a debt (which we haven't a clue as to what it is). The 1st phone call my husband received and they were very rude, therefore he hung up and won't answer the phone. They leave message after message, day after day and won't stop. Well now they are calling me at my office and on my office cell phone and won't stop. Like him, I have gotten to where I won't answer the phone when I see that it is them calling. Today I came home and found a FEDex door tag on my front door. I tracked it and guess where it is coming from NCO in MD. As of 2 weeks ago our credit reports were fine, (we just purchased a brand new car), but now, they are going down the drain because of these idiots. I don't know what to do or where to go for this. Of course, I have the letters from the companies and we have contacted the Credit Bureas, but I don't know what to do about NCO. I won't just hand this information over to them, as all of our personnal information is on these letters. Please help me as I am at a loss and am severly frustrated with NCO.

Tired of phone calls
Houston, Texas
U.S.A.