Since when did this become redditard writing prompts?
The answer is simple, you call in a tip to the FBI hotline. You describe how the attackers intend to take down the buildings, how the planes themselves will be deliberately crashed, that they will kill the pilots and that they have enough pilot training to steer into the buildings themselves. That each plane has 4-6 attackers, they intend to use blades or boxcutters, that they believe the passengers won't fight back because they'll mistake it for a hijacking. Given the flight numbers of each of the 4 planes. Mention that the targets might well be variable if they take longer to get control of the cockpit. You could even memorize the attackers' names, but you would want to point out that there may be backups/sleepers you don't know of. If you rehearsed this, the message could be under 30 seconds long, wouldn't even have to talk to a person.
For bonus points, accuse the Mossad of knowing about the plot but that they intend to keep quiet to try to steer the United States into war in the Middle East.