This is what's known as "answering the wrong question". Even if it were possible to do this in real time - which, of course, still hasn't been demonstrated - there's still the problem of what the phone calls were actually saying. To fake this, the conspiracy would have needed not only personal information on the personal background, typical vocabulary and speech patterns of the people it was imitating, but in one specific case some even more specific information.
Linda Grolund, aboard UA93, spoke to her sister Elsa Strong, and told her that her will was in her safe, which was in her closet. She then told her the combination of the safe.
How are the conspirators supposed to have known that information?
Dave