Some of the people in the World Trade Towers were able to call out and say their goodbyes to their loved ones before they died. Others left messages on answering machines and others spoke to 911 Operators.
Here are some links to a BBC Documentary called, "9/11: Phone Calls from the Towers"
[Part One] [Part Two] (embedding has been disabled by the uploader)
More information about the film at The Mirror.