Those are usually robo dialers looking for active numbers to resell to spammers/scammers. You answering puts you on their good list. These are also the calls that never leave any type of voicemail. I’m not sure what list VM gets you on.

This sounds intuitive, but isn't true in my experience. It's a natural consequence of aggressive dialing with a limited pool of agents. See my sibling comment: https://news.ycombinator.com/item?id=40882163

> I don’t understand what they are calling for either. I’ve answered a few and most of the time it’s a dead line when I answer. Just silence.

The primary operating goal of a predictive dialing system is minimizing agent downtime. Ideally, when an agent transitions into being ready to talk, they want as little time as possible before they're connected to a live lead.

In above-board telemarketing, where there's a finite list of leads instead of 000-000-0000 through 999-999-9999, the administrator will adjust dialing aggressiveness to minimize the chance that a lead picks up the phone but no agent is available to take the call. Because when that happens, the answering party experiences nothing but dead air, followed by a timeout, and a hangup.

The one nice consequence from this, though, is that if you do answer a spam call and get connected to a live person, chances are very high that several other potential marks got dead air instead. Maybe you saved grandma for another day.