Even with the voices of the callers stripped from the tapes, the horror witnessed inside the towers is painfully clear.
"OK. A plane just crashed into that building," a dispatcher told a caller on Sept. 11, 2001.
"This is at the World Trade Center, sir," another told the dispatcher supervisor. "I have a woman that's hanging from the building from the northwest side of the One World Trade Center."
With smoke filling both towers, countless callers simply wanted to know how to keep breathing.
"Stay calm," one operator said. "I agree with you that you need air, but I can't tell you to break a window. Now, I can't tell you if you break the window you might let more smoke or debris in."
Other dispatchers, sensing their callers' desperation, said go ahead and do it.
"Floor Number 88, Building 2," an operator told a stranded caller. "Do you have a heavy smoke condition? Any windows open up there? Break the windows. Break the windows."
Many callers wanted to know if they should stay or try to get out. Police and fire commanders on the ground had already ordered evacuations after the first plane hit.
That message never got to dispatchers at 911.
"I believe they should remain where they are," another dispatcher said. "If they can, put something to block the smoke from coming in under the door, because we are in the building and the hallways are filled with smoke. They should not go in the hallway."
"OK. All right? They've already started breaking the windows," an operator told the supervisor. "They are panicking on that floor."
It was a breakdown in communication exposed by the 9/11 commission. Brian Clark remembers calling from inside the South Tower and being shifted from dispatcher to dispatcher.
"I actually told the same story to three different people," he said.
David Rosenzweig, president of the Fire Alarm Dispatchers Benevolent Association, oversaw dispatchers at the Fire Department that morning.
"We sensed their panic, and we had to change more or less … our directions to them because they were panicking," he said. "We knew they were having difficulty breathing. They couldn't survive in the halls and the stairwells. The only other alternative was to break a window."
Dispatchers knew their callers were running out of time.
"I understand, please," an operator instructed a caller. "Stop talking, and let the air. … You're losing your oxygen. So try to be quiet and remain calm. OK? Please."
Some dispatchers offered to call loved ones -- knowing so many might not get out.
"Do you have a phone number to your home that you would like for us to call anybody?" a dispatcher asked.
Among themselves, the operators discussed how they dreaded breaking the news to family and friends.
"Oh, God," a dispatcher said. "And it's an awful thing. It's an awful, awful, awful thing to call somebody and tell them you're going to die."