Australian radio personalities Mel Greig and Michael Christian later apologized for the hoax — sheepishly noting that they were surprised that the call was put through and that their Australian accents were not detected.
William's father, Prince Charles, poked fun about the incident with reporters on Thursday when he said he was very glad to hear the Duchess was feeling better.
"How do you know I'm not a radio station," he joked, later adding that he is "thrilled" about the pregnancy.
"It's a very nice thought to become a grandfather in my old age, if I can say so," Charles said at an event in London.
The royals have been the target of hoax callers before. Canadian disc jockey Pierre Brassard telephoned the queen in 1995, pretending to be Canadian Prime Minister Jean Chretien.
In a conversation that lasted 15 minutes, Brassard managed to elicit a promise from the monarch that that she would try to influence Quebec's referendum on proposals to break away from Canada.