Didn't answer this number because I didn't recognize it, and they left no message.  When I called back from an unlisted number, I get a voicemail saying "You have been contacted because you have been selected to conduct a survey.  We may try you back later...." and then goes on to say you can leave a voicemail.  Even if this a legit survey (which is doubtful seeing as they state no company name in their voicemail when you call the number back), who actually considers landline surveys an accurate representation of the mass opinion?  Blocking them.