This number called. CID said "not in service," so I assume it was spoofed.  Seemed like a well done robo-call.  "Mike" told me I could qualify for a great new security service being installed in my area. He'd have to ask me a few questions to see if I qualify.  First, "Are you the home owner?"  I said "I have a great security system I'm happy with."  After a pause, "Mike" said "You qualify!  I'm putting you through to a specialist. Don't hang up!"  I hung up.  SCAM.

Called but didn't leave a message. The CID of "Not in Use" at least has some humor.

Same as previous reports. CID recorded "not in use".

I answered after 5 rings, somebody or some machine said "hello" and I waited and apparently nobody was home.  May have been a machine or a human but he/it didn't respond further...

This sounds like the newest robocall scheme using a near-human "robo-response".  The "tell" is if you answer with an unanticipated response, it responses differently than a human would. In this case, if you had responded with a "yes" you probably would have been asked more questions. Or if you had responded with "no" you probably would have been asked to get the home owner.

You answered with "I have a great security system I'm happy with." If you had said, "My bucket has a hole in it," or "Bwa li kote solèy la pa klere" (Haitian Creole for "Stick it where the sun don't shine") you still would have been sent to the specialist.