You should really just press 9 right at the beginning of the robocall. Most of the robots use that as a way to automatically put you on their Do Not Call Lists. I sat through the first few when they first started robocalls to figure out the menu structures. In the beginning it was a mix of 9 and 2 for the robocall DNC, but I think they've mostly standardized to 9 and I haven't been getting calls. I did recently get a Spanish robocall at work that used 2 instead of 9 for their DNC, but once I figured that out, I've stopped getting them.
The original way to stop calls was to ask for their name, company and call back number for the company. Then you asked to be put on their local DNC list and note the date somewhere. They are legally required to keep on their list for a year. If they call back within the year, you have the right to take them to small claims court for $500. They usually won't ever bother removing you from their lists because it's too much trouble and you're likely to ask to be put back on the list. They also don't want to go to the trouble in case they did mess up the date and you do happen to sue them outside their state. You just need to put in 1-2 month of effort in actually answering the call and the calls will stop. It's much easier with a robocall; just push 9 as soon as you hear the robot and it hangs up immediately and you don't have to hear the whole spiel.
The National Do Not Call list is a scam by the politicians. It was created for the stupid people that don't understand that there was already a Do Not Call Protection in place or the lazy people that didn't want to put in a tiny amount of effort to stop the calls. Putting yourself on the DNC list puts you on the political call lists. You've given them a valid number to look up. They put in an exemption for themselves. I know some people that did used the DNC list and they're now getting nuisance calls during election season.