I bought a London policemen's whistle when we were there years ago. One long blast on this and they DON't call back. Worked real good for an ex s-i-l that called at 2am the second time.
Answered first time because it was a some-what local # - person answering from first word spoken did NOT sound proffessional (Hang up!) on DNC list so thought maybe a legit call - thet kept calling, no messages ever. Called # back couple days later after repeated call, almost choice option 2 but decided to "Google" and found this site - glad I did. Just did a # block on my own phone.