Prince William was 15 and Prince Harry just 12 when their mother was killed in a car accident in France. Now the brothers are opening up about their biggest regret: their last phone call with Princess Diana. In the HBO documentary film, "Diana, Our Mother: Her Life and Legacy," the princes remember their mom as a prankster. They were vacationing in Scotland when she died and, in the movie, they recall their last phone conversation with her. #InsideEdition