Either their robo calls got stuck or they are listening to see if there's a response. Best thing to do is never pick up strange calls and block them with a call blocker.

This number keeps calling my cell phone number, I can hear background noise, but the caller never says a word and just hangs up, when I call the number back I get a voice mail message or I am told this number doesn't exist.  I'd love for this harrassment to stop!