NCO collection agency has been into harassing people for many reasons. On such occasions, you are required to send a cease and desist letter to stop all telephone calls and other modes of correspondence. They are required to stop after receiving your letter, otherwise, it will be a violation of the federal law.
If you have a debt with them and you are regularly paying towards your account, they cannot take any sort of actions against you and should not even disturb you in anyway. If they do so, you should lodge your complaints against the company for unethical and harassing business tactics with FTC, state attorney general office and the BBB.
At this time, it will be preferable to contact the original lender and see if they are still holding your account. If the debt has been not sold to NCO, they cannot charge interests on it because you have not put your signature on the contract with the collection agency.
Pennsylvania-based NCO Group has been the most horrible company in the books of FCRA. They have been doing the illegal activities by re-aging the accounts of the consumers and not reporting the actual dates. Due to this, the seven years reporting time of an account gets extended due to illegal activity. Federal law states that a violation of this type is subjected to FCRA laws and the company has to pay the penalty charges of $2500 per violation.