set financial. if you or someone who owned the phone number contacted you, this would be the reason why they call. If you are not the person they ask for, let them know. If they continue to call despite your response, file a cease and desist with the company.
If you are in fact the debtor, call them/answer them to resolve it. See if they can accept payments.
With any debt, try to reason with them. If not, get legal assistance. Goto http://www.consumer.ftc.gov/articles/0149-debt-collection for tips. Download the law by going here >http://www.federalreserve.gov/boarddocs/supmanual/cch/fairdebt.pdf