Same here. I think they are phishing calls. In many cases if answered they will use foul language or just hang up. Active numbers mean they can sell it to telemarketing companies. So, please do not answer if you don't know who is calling. A reputable company will always leave a message on the answering machine.

Could be the "duct cleaners" again.  Leaving the phone off the hook,  maybe they'll get tired of hearing a busy signal.

World Wildlife Federation asking to speak to my Grandmother who's been dead for 5 years.

Must be calling to find out if the phone is ringing.  Time to get out the straitjacket :)

Got a blank voice mail. Could be phishing. Do not answer.