I am sure these people are smarter than just using a single number. They can just use random number every time.

You could for example have a white-list but that's won't quite work for most people.

They could use a random number every time, but they don't. If they start doing that, I'll enable Google Voice's feature that requires an unknown caller to say their name before the phone call is accepted.

That's pretty cool. Actually my friend implemented a feature on his home phone (he is old school) that had a recording answering the phone and it said, if you are human press "8". If they did then the actual phone would ring, otherwise I guess I just hung up or just kept them on the line. So he screened all the pre-recorded crap.

I don't live in USA so the question might sound stupid. Are there 100% automated calls coming from marketers? What are they saying... do they just tell you about a product or..?

Yap.

"We are calling to talk about your car. Do you know that you can get a lower car insurance in your state. There is some new law that lets you blah...blah...blah. Press 1 if you would like to sign up."

Or

"Do you know Obama is from Mars. He has an Alien living inside of him and telling him what to do. Do not vote for Obama"

This is very common for political campaigns as those are also except from some laws that a design to stop these.