Have you received a bill from an unknown company? If you default on a credit balance, creditors have the option to sell that account to a debt buyer. These are not collections agencies. The original creditor has sold your account, often for pennies on the dollar, to another company. Therefore, a debt buyer is not in the business of collecting a debt on behalf of someone else. You now own them the money.
Debt buyers will buy thousands of bundled accounts at one time, well below face value. Their bet is that they can collect enough money to make a profit. So, they will send out notification letters, demand payment, and sometimes seek judgments and wage garnishments. The original creditor no longer has anything to do with the balance and the debt buyer makes all the decisions.