calls everyday & leaves no message

I have had five calls already to day from this number. No one is there when i answer it. I tried calling the number back but got a recording that said sorry, your call cannot be completed. Does anyone know what is going on with this number? I do know its a land line.

I was told they call different times of the day to see what time is the best time to get in touch with you.
Was told it is an automatic call, that records the times you actually answered the phone, so then a real person will call after they compare how many times you answered the phone at a certain time. That is why no message is left.

Get daily unwanted calls, no one on the line when I answer