Didn't leave message. Called them back later, it said survey was over, pressed 9 to be removed from calling list; after a pause, it said my number had been removed.
"They" called my Do No Call mobile, left no VM. When called back with *67 prefix, heard very poor quality low bandwidth recording that "survey was over" and offering to be removed from their call list by pressing 9. Pressing any key or "9" results in disconnect. Doing nothing, results in disconnect in about 10 seconds of end of message. Likely if this number was called repeatedly several times a minute, 24/7 it would interfere with scammer's ability to scam others. Programming a fax machine to do so, with a fax message to "cease & desist" calling numbers on the DNC list would be helpful.
Update to above. Warning, pressing "0" repeatedly restarts the recording repeatedly. Surely no one would want to tie up the scammer's system by doing that, so just thought everyone would want to know.