I suspect I've deal with these asshats.
My old phone number was previously owned by a couple who obviously hadn't been paying their bills (hi Paula and Ricky!). I got called up by debt collection agencies looking for them all the time. I'd have to go through the spiel of how I wasn't the person they were looking for (not like they ever believed me). Sometimes I'd get called repeatedly, sometimes they'd promise to take me off their lists.Then a few weeks later, I'd get called up by a different agency.
I can only assume that the debts were bought in bulk, the agencies made a few half-hearted attempts to collect, then sold the debt on (I thought that there was an argument that once a debt was sold on, it was considered fulfilled, but perhaps not).
The weird thing was I got wrong number calls for this pair all the time from their friends too, so I have no idea what they were up to. Glad to be rid of that number, though.