NCO Financial is a collection agency. A Google search will turn up a plethora of complaints and incidents involving harassing and/or illegal tactics. If these people call you, demand that they send you something in writing that includes...

1 - Name of their client to whom they claim you owe money

2 - Amount owed

3 - log of transactions against said account.

It is my experience and opinion that these people will. at times, outright lie. Demand that they deal with you in writing only and they tend to be somewhat more honest.