I'm not sure if it is coincidence, or if it is really helping, but I play SIT code to indicate a number that is not in service at the beginning of my voice mail message. Since then, the number of spam calls I've been receiving has been steadily decreasing.

Install sox

play -q -n synth 0.2 sin 950;play -q -n synth 0.2 sin 1400;play -q -n synth 0.2 sin 1800