Do not provide any personal information. If the debt is not yours, ask the caller to provide all the identifying information they have over the phone to verify whether they have your information, or are just following up on similar names. Even if they have information that is yours, do not provide more information. Always make them tell you what they know. If they provide information that is not yours, simply state that it is not your information and politely end the call. If they persist in calling you, there are local agencies you can report them to.
If they have your information, then ask for all of the details of the debt -- who is it owed to, when was the debt incurred, what was the original amount of the debt, what is the current balance, when was the last activity on the account, what is their relation to creditor.
Once you know the creditor, you can contact them directly for more information. It is possible they may have written off the account and closed it, selling it to a debt collector in order to get some sort of return on debt.
If they truly have a debt that is yours, and you did not incur it, then you will need to file a police report for a case of identity theft. Be prepared for some scrutiny.