NCO is often hired to come in and collect a debt for a lender they also will purchase debts and collect for themselves. They have a reputation for ruthless collection methods that often run afoul of the law.
In 2004 the FTC imposed a $1.5 million dollar punishment on NCO. NCO had been exploiting the credit system. They were reporting debts late in order to keep the debt on the borrowers' credit report for more than 7 years. The fair credit reporting act says that a delinquent debt can by law only remain on your credit report for 7 years.
Collection agencies are known to have called a borrowers family member, neighbors, places of employment in order to collect payment. To stop a collection agency from contacting you in the future you need to mail a letter where you say you do not want further contact from them.
I strongly recommend sending this letter certified mail, so you have proof that it was sent. Also fax a copy and save all documentation of any communication between you and the agency. This will help when filing a complaint or filing a lawsuit against the collection agency.
When this letter is received the collection agency will be able to contact you once more in regards to how they will move forward with collecting the debt. This is the law according to the Fair Debt Collections Practices Act of 1996 which was made in response to overwhelming complaints of unethical collection methods by collection agencies.
The Fair Debt Collections Practices Act says debt collectors can not misrepresent themselves or tell anyone else about your debt other than you and your attorney. Typically this piece of legislation is completely ignored and collection agencies frequently go unchecked.
Congress passed the FDCPA to protect individuals from debt collectors. Agencies are frequently found in court facing lawsuits and have become very crafty at avoiding these. They will change their operation name and have been known to hide assets to avoid paying on a lawsuit.