I couldn't verify this officially, but this is what I've heard (and I believe it):
It is telemarketers. They have an automatic systems that call numbers at a certain time of day to see if the owner answers. I can believe this because before I registered on DMSA's opt out list, I would get a call with nobody on the other end and the next day or the day after at roughly the same time, I'd get a call from a telemarketer. Since I've opted out I have not had a single call of that nature (or from telemarketers).