If you don't have credit issues, is it possible that you have a friend or family who does, that at one time listed you on their credit applications as a reference? I have had that happen before, where I was relentlessly pursued by a creditor looking for my ex. I never co-signed with him on anything, but he had listed me multiple times on applications as a reference. You would be surprised how easily creditors can get a hold of all your personal information- when I stopped picking up the phone for them, they somehow got a hold of my father's office number and started harassing him!
The next time they call, try doing *69. It should work as long as they aren't calling from a blocked number. Or get access to your phone records from your phone company and see what number keeps dialing in around those times you get those calls.