unknown caller, let it ring, hang up

called my landline I answered then they hung up on me.
More crappy robo calls

received call several times in past couple of weeks; no message but I heard background that led me to believe it was a telemarketer of some type.  I called the number and received a message indicating 't=Thank you for your call.  I'm sorry no one is available.  If you would like to be added to our do not call list please press 1 now or if you would like to leave a message please press 2 now".  I am already on the national do not call and so I should not need to be added to their do not call.  I hung up.

Have they called back since. I too would like to get rid of these people.

I'm getting a lot of these types of calls - from all over the country.  I get the feeling these robo calls are intended to confirm someone answers and just by calling back or picking up phone gets your number on a large mArketing list. It's just so weird with so many in just the last 2 months.   We are the Govt controls?  Blocking appears not to work except for a specific number since they call with millions are area code combinations.