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How to get rid of someone else's debt collector?

I've recently started getting bill collector calls regarding a debt for someone I don't even know. I assume they have the wrong phone number on file or something. Of course, I'm sure they always hear that when trying to collect debts and probably don't believe it, and inevitably this will get passed through two or three collection agencies if the original person isn't found. I'm dreading a long series of calls where I have to explain over and over that they are calling the wrong person.

What can a person do to shake off a bill collector that is barking up the wrong tree? Do they have any legal obligation to fix their records and/or stop calling me when I tell them they have the wrong number? I know there are all kinds of rules debt collectors have to comply with when dealing with the actual person who owes the money, but do I have any protections as an unrelated third party?