Caller ID!
That’s my reason. If I don’t recognize the number I let them listen to the answering machine message. If they don’t want to leave one, I don’t want to talk to them.
Perhaps 80 or even 90 percent of the calls coming into my phone are spam of one kind or another. Now, if I hear my phone buzzing from across the room, at first I’m excited if I think it’s a text, but when it keeps going, and I realize it’s a call, I won’t even bother to walk over. My phone only rings one or two times a day, which means that I can go a whole week without a single phone call coming in that I (or Apple’s software) can even identify, let alone want to pick up.
More @ Why No One Answers Their Phone Anymore