FWIW, is that there was rattling noises which you get if you don't get the receiver back on the cradle. On certain phones, there are two buttons that need to be pressed down in order for the phone to be hung up. If only one is pressed down, then the person on the other end can hear the clunking of the button, but effectively the line is still open. Other phones have one button, but it needs to be depressed for a second or two before it cuts the call off.

I understand from Thomas' description of the call that Patsy didn't get the receiver back on the cradle first go (understandable if she was shaking badly) and the operator would have heard the clunking noises and perhaps also the voices in the background. As the clunking noises were closest to the phone, they would be the loudest and would have fairly drowned the voices out. Anyone listing to a first generation copy of the tape would hear the clunking/rattling noise of the attempts to put the receiver on the cradle and nothing else.

By running the tape through a spectrograph forensic tape analysts can delete or obscure non-verbal content. What is left can then be amplified.

*****
Similar scenario
Earlier this year, the FBI enhanced a photograph for the British police working on the case of missing schoolgirl Milly Dowling. It was a CCTV image which was blinded by sunlight. In the original image there were some blurry images which could have been cars. Enhancement showed a figure beside a car, but anyone could look at the original image and say "thre is nothing there" and who could argue?

Allowing people to listen to the non-enhanced tape will prove nothing and speculation will continue.