Moreover, the Gestapo did NOT have any means of breaking the British/U.S. codes and Churchill and FDR did not talk on "open channel," nor by radio or telephone at all. It is pure bunk.
To quote p. 22 of the book: "This [the facts that the calls were scrambled and the two men were careful about discussing any sensitive matters] did not prevent the Germans from intercepting some of the communications and making transcripts. In 1943 a special intelligence unit of the German Postal and Telegraph Service intercepted a call that Churchill placed to Roosevelt to discuss the armistice they were secretly negotiating with Italy -- 'incontrovertible evidence, the Germans reported, 'that secret negotiations between the Anglo-Americans and the Italians are taking place.'"
In fact, Churchill phoned Roosevelt the evening of Pearl Harbor day, Dec. 7, 1941, to get confirmation that the Pearl Harbor attack had occurred. See pp. 114-15 of the Stafford book.
I think it is unlikely that Churchill would have given this particular sensitive matter of an impending Japanese attack on Pearl Harbor to FDR over the telephone, but I would certainly like to see the evidence. What I have always understood was a written message went from Churchill to Roosevelt just before we presented what was in effect an ultimatum to Japan in late November 1941. This is the only piece of the Churchill-Roosevelt correspondence that is still being kept secret. It obviously must have contained something extremely sensitive or embarrassing.