Called. Unknown caller. Left no message.

They called and I answered. Automated (I think) message played asking me if I wanted a free security system. I said no thank you.

Could be a telemarketer or it could be that scam where they try to get you to say yes.

answered (no "hello") they dropped without talking

They have called me several times today on my two phones. Relentless.they just hang up.

Right now they called five times in the past six hours trying to sell a security system.  They repeatedly called of my both cells and house phone indicating that some online company sold information to said scammers.