Knew it was a most likely a spam call as we receive dozens of them to our office each day.  Always disguised as a local number, always a long pause after picking up the phone.  At this point I pick up but say nothing and leave the phone off the hook until they decide to hang up.  Tried ignoring them in hopes they'd think the line is inactive, but see the same numbers call every day regardless.  At least now I'm tying up their line for a few minutes each time they call.