You are probably the only one who can make them "go away."
First, stop answering when they call, if you are still answering. Why would you, though?
Second, don't bother pressing 9 or 1 or anything: You know it doesn't work. Why keep doing it?
If you're there when this number shows, pick up and hang up immediately. Don't be tempted to say anything and don't wait for anyone to say anything to you.
Eventually the calls will stop. If you keep pressing numbers, you keep validating your phone as a live phone with someone who is willing to "play the game" with them.
First, since you have a business phone, the Do Not Call list does not apply to you. It applies only to personal phones.
However, you STILL have options!
The best one is to get a phone blocking service, app or device.
Then, learn how to use it.
You will be able to prevent SECOND (or greater) calls from scammers, after a simple button push or two.
Google "phone blocker" or "call blocker"
I'm sorry, I guess I wasn't clear. This is a number that calls every month. It's an easy number to remember (I looked at it once and have practically got it memorized.)
When this number shows up on your caller ID, that's when you can choose what to do.
If you can't use a call blocker, IMHO a simple post-it note with this number and maybe one or two others, stuck right on your caller ID panel, should remind you not to answer this call, or to pick up and hang up immediately, with no other interaction.