So What's the Deal Here?

I figure someone who reads this site can probably explain it…

About twice a day, sometimes more often, I receive a mystery call on my cellphone, always from the same number. The area code is 866, which is a toll-free area code. The first three digits of the number are 869. Googling the entire number tells me thousands of people are having the same experience with this number. It rings. You answer it. There's no one there. And two seconds later, it disconnects.

There has been one exception. Today, I answered it. I said "Hello" and I heard a man's voice. He seemed a bit startled, like he hadn't expected anyone to answer. Then…

HIM: I'm sorry. I seem to have dialed the wrong number.

ME: You seem to dial this wrong number several times every day.

Then I heard him start talking to someone else and the line went dead. Five minutes later, the number called me again with no human attached.

Googling tells me it's some sort of marketing survey company but no one explains what they're surveying or how. So far, all they seem to know is that when they dial my number, I answer. Do they really need to test this premise twenty times a week?

What am I missing here? How is it of any benefit to them to place those calls to me? Someone here will know.