If it's a scam, it a law enforcement issue. Tell the cops. Even if they don't help, you've got a paper trail. Your state's Attorney General's office may have a division to investigate consumer frauds and scams, as well.
There are some other possibilities. First off, are they asking for you specifically? There's always a possibility that they're a junk debt collector that's trying to collect on something that's actually been paid off, cancelled or otherwise dealt with that another debt collector with (deliberately) shoddy record-keeping sold them. Could also be an actual debt, but one you shouldn't really owe because it was fraudulently or accidentally applied to you instead of someone else. This can be a real PITA to sort out.
If they're not asking for you personally, it may be someone else's debt. I've had bill collectors call me repeatedly because someone who had my number before (allegedly) owed someone money. Convincing them they got the wrong guy took many tries, and sometimes they just sell off the debt to another junk collector who'd try again. One guy even asked if I knew the new number of the (alleged) deadbeat. Really? It's a phone number -- It's not like I sold someone my car. Those calls only went away with a change of number when I moved.
Unfortunately, Fair debt collection practice laws don't seem to be as helpful to people who don't actually owe a debt. I guess the authors really didn't expect that issue to come up, but it does.