You, too, huh. Usually, we don't answer. We keep telling them we don't know the person and to quit calling. They'll stop for a few weeks and then it'll start back up again. Apparently, the dead beat made up a phone number which happened to be ours.
In our case, I think my number was the deadbeat's number, although at a different address. As for the start/stop--what happens is that they resell the debts to other collectors for ever lower percentages. The new owners call until you tell them you never knew the person and have been getting these calls for years, and then stop. Then they sell the debt to someone else, and it starts up again. It's slowing down, though.