Get a clue, okay?
If you call someone's home phone number a half-dozen times over the course of a week, sometimes during the day, sometimes at night, and you never, never get an answer, which of the following possibilities do you think is the most likely scenario:
a. Nobody's ever home at this number, but the people who pay for this phone would really, really like to participate in your survey and would feel terrible if you didn't keep calling back until you finally reached them; or
b. The people who pay for this phone have Caller ID and have no intention of talking to you, no matter how many times you call back; or
c. There's a remote chance that one of the people in the household you're calling might be sitting right next to the only phone in the house that doesn't have Caller ID and might be expecting a call and might reach out and answer the phone without screening it first.
Crap. The correct answer was "b," but you picked "c," didn't you?