The book is pretty obscure when Carlo is talking to Hagen: "The tension in Hagen's voice seemed to have finally alerted him that the news coming up was going to be really important," doesn't seem like Carlo knows all. It's always seemed unlikely to me that Carlo would have a girl call the apartment, it just adds his philandering to everything else. Maybe all Carlo did was tell them Sonny took the Causeway, and all the rest of the plan was a surprise to him, too.