It's the Red Cross, requesting blood donations. I did speak with someone. It's an auto dialer, which is why sometimes there's no one on the line when you pick up. It will keep auto-dialing multiple times per day/week, until you speak with a live person & they can move the next contact date.
This number has been calling my husband every day for the last week without leaving a message. This morning was the first time it called me without leaving a message. I blocked it on my phone so let's hope it stops for good now.