IF YOU WANT TO KNOW WHO CALLED AND REMAIN ANONYMOUS, DO THIS;
PRESS *67 and wait 3 seconds for the beeping sound to stop, and then dial back their phone number.
Doing this displays 'PRIVATE' on their ID Caller display -- they cannot see your number. Chances are you'll get a recorded message OR a message that says it's not a working number. Don't leave a message.
Next time these pricks call, give them a chance by telling them to STOP CALLING and to take you off of their call list immediately ... and then hang up. Don't let them suck you into a crazy conversation.
If they call back anyway (and they probably will) then it's time to;
BLOW A REAL LOUD WHISTLE INTO THE PHONE WHEN THEY COME ON THE LINE. Works every time.