My approach is to answer the call but ........say absolutely nothing! Complete silence.
. I stay on the line until the caller hangs up. With no verbal response, the caller or computer thinks it is a "dead line". Therefore, no more call backs
I read about this in the AARP magazine as a way to cut down the number of annoying calls.
It may take a little time to realize that your phone is not ringing as often, but for me, it has worked.
Try it..... May work for you too.