After a long pause when I answer, a woman named Julie answers. She immediately asks "can you hear me OK?" In previous versions of her calls, she says "Oh, sorry, my headphone wasn't working, can you hear me OK now?" Very bubbly-sounding voice, but obviously fishing for my voice saying "yes" or "no." Creepy. Number blocked.

They are scammers. They called a number on the Do No Call list, I didn't answer. I blocked my number and returned the call to get a message saying that the survey I was called for has ended. I've gotten no less than 50 of the same type of call from different numbers - pretending to be a survey to get around calling numbers on the DNC list.
That in itself is an indication that they're disreputable and likely scammers. Other sites indicate they're offering vacation packages.