I wait, talk to the human, and help them to realize they have the worstest job in the world. Then I add them to my cell phone, but mark the SEND DIRECTLY TO VOICEMAIL box. A few choice words describing her [***] helps too.
I received a call from this number earlier. I also called the number back (from an unregistered prepaid cell phone), and received the same message ( "the number you are calling has a voice mail that has not been set up yet."). Another phone number that will take its final (as far as I'm concerned) voyage to "Block City".