"Gotta be small," he says.
A few seconds later the second plane hits and the conversation takes a dramatic twist.
"Oh, whoa, whoa, whoa," he says. "That didn't feel good."
With hindsight we know it as a turning point in international affairs, a moment that will be endlessly replayed in words and pictures and both invoked and evoked by politicians in the name of war and peace.
But what the transcripts of phone calls released by the New York port authority late on Thursday reveal is the gradual awareness of those in the towers and those rescuing them that this was no straightforward emergency.
Families of the victims were divided over the transcripts' release, ordered by a New York court at the request of the New York Times. The horror, heroism, tragedy and shock of those involved are played out in real time - showing some of the mistakes that were made but also a great deal of bravery.
They show that while some played it down, others overreacted. A man identified as Tommy called his mother and warned her. "Just stay in. Don't do nothing. This is bad. They got planes all over the radar coming into New York area. They think everything is going to start hitting."
As they realised this was an irregular situation, regular people were forced to make extraordinary decisions.
These are some of their stories, which have emerged from the transcripts.
Windows on the World
Christine Olender, the assistant manager of the Windows on the World restaurant, had gathered together all the breakfast guests and employees on the 106th floor of the north tower when she called the Port Authority 15 minutes after it had been hit. The three emergency stairwells were full of smoke.
"We are getting no direction up here," she said. "We need direction as to where we need to direct our guests and our employees as soon as possible."
The police officer gave what solace he could.
"We're doing our best. We've got the fire department, everybody, we're trying to get up to you dear," he said, asking her to call back in two minutes.
"Call back in two minutes. Great," says Olender before hanging up.
But she was nothing if not persistent, calling back four times in all to receive police instructions.
"Hi, this is Christine, up at Windows. We need to find a safe haven on 106, where the smoke condition isn't bad. Can you direct us to a certain quadrant?"
She was told the emergency services were on their way.
"What's your ETA?" she asked.
"As soon as possible. As soon as it's humanly possible."
With no help in sight those assembled on floor 106 were being smoked out. Olender called for the last time. "The fresh air is going down fast! I'm not exaggerating."
"Ma'am, I know you're not exaggerating," said the officer. "I have you, Christine, four calls, 75 to 100 people, Windows on the World, 106th floor."
"Can we break a window?" asked Ms Olender.
"You can do whatever you have to to get to the air," said the officer.
Waiting in vain
At a Port Authority office on the 64th floor of the north tower, civil engineer Patrick Hoey called police dispatch in Jersey City for advice.
"What do you suggest?" he asked 25 minutes after the first plane hit.
"Stand tight," said the sergeant. "Stay near the stairwells and wait for the police to come up."
"They will come up, huh?" Hoey asked. "They will check each floor? If you would just report we're up here."
About an hour later Hoey and his colleagues were still there, with the air short and smoke rising. "The smoke is getting kind of bad," he told the police desk. "We are contemplating going down the stairwell. Does that make sense?"
The voice on the other end of the line was now less confident that help would be at hand. "Yes. Try to get out," says the officer.
"All right. Bye," says Hoey.
The boss's wife
Christie Ferrer was searching for her husband, the executive director of the port authority, Neil Levin.
"Hi, I know you're crazed," she says. "I don't want to bother you but the governor is looking for Neil and so am I and no one can find him. Did you guys locate him?"
"I haven't got that information. But the last I heard he was not in his office. Hold on a second."
Ms Ferrer is put through to World Trade Centre director Alan Reiss.
"I really don't know where he is," says Reiss.
"Do you know for a fact that he wasn't in the office?" she asks.
"I don't know that for a fact," says Reiss.
Levin's body was found months later.