If you’re referring to the September 11 incident, it’s my impression that everyone pronounces it “Nine-eleven”, since the day was “eleven”. But if you’re calling for an ambulance or fire truck or police in the U.S., it’s my impression that everyone now pronounces it “Nine-one-one”. My recollection is that people used to say “nine-eleven” for the emergency number, but that that pronunciation has declined, perhaps because of confusion with the “nine-eleven” incident. Am I incorrect? I don’t listen to radio or television, so my sample is restricted.
Edited to clarify question.