If you are contacted by a collection agency regarding an unpaid account, You, as a consumer, have certain rights as outlined in the Fair Debt Collection Practices Act (FDCPA) http://www.ftc.gov/os/statutes/fdcpa/fdcpact.shtm, Further, collection agencies are prohibited from using abusive, unfair, or deceptive practices to collect from you.
In recent years there has been an increase in scams claiming people owe unresolved debts. The debts are sometimes associated with payday loans. The caller will claim they are associated with the loan company, a legitimate collection agency or a law firm. They will then use the threat of being arrested to intimidate their victim into giving them money. Failing to pay a loan is not punishable by arrest. In fact, threatening to have someone arrested for not paying a debt is illegal.
If you are unsure whether you are delinquent on an account, request the caller to validate the debt in writing. If the debt is legitimate the collection agency will have no problem forwarding evidence of the obligation. Once you have received evidence that the amount is owed you can then make arrangements to resolve the obligation.