I have a couple of phone lines, all of them unlisted. Around once a week, the following happens. One of my lines rings, I answer it…and after a beat, I hear a polite woman's voice say, "Oh, sorry…I think I have a wrong number." Then it disconnects. This has happened at least a half-dozen times and I now realize the woman's voice is recorded. It is not a person placing these calls but some kind of computer.
I'm guessing this is some phone-solicitation company mining for phone numbers; that they have a computer dial every possible combination — or perhaps every possible combination that is not already assigned to a listed number. If someone answers, it adds that number to a database from which phone solicitations can be made. That way, they don't sell their customers numbers that will turn out to go to fax machines, modems and other numbers which won't be answered by anyone who can buy stuff.
Does anyone have another theory?