As far as I know, this is the first (and only) recorded excerpt available to the public which reflects communications between one of the flights on 9/11 and someone on the ground. Recently posted at the Memory Hole, the tape is Betty Ong (flight attendant on Flight 11) calling the American Airline Reservation Desk.

In an abundance of caution, I think I should warn that the tape may not be suitable for everyone. It’s not gory or anything like that, but unsettling; for me, hearing the tape brought back that day in a profound way.