I answer and hang up. That way they can’t leave a voice mail.
If you have Verizon, they have a call filter app. You can pick the types of calls you want blocked.
My phone is supposed to be able to filter calls by just the area code and THAT doesn’t even work.
“I answer and hang up.”
And now the call-bot knows the random number it just dialed is a good, working number. It will soon be added to the list of known good phone numbers for sale.