The Phone Booth of the Future was a phone booth made for the Hill Valley Science Expo of 1931. In addition to giving the person inside privacy, it allows for the caller to speak without using their hands.
The voice for the Phone Booth Operator was provided by Rebecca Schweitzer.
During Marty McFly's quest to save the future Hill Valley from turning into a police state, he needed to discredit one Edna Strickland. To do this, he went to the Future Furnishings exhibit, where another phone resided. He called the Phone Booth of the Future and asked to speak with Edna. Impersonating Carl Sagan, one who was conspiring with Strickland to keep the future Marty was trying to save from ever happening, Marty managed to get Strickland to confess that she was the one who had burned down several speakeasies.
Having placed a secret recording device in the booth beforehand, Marty, after the call had ended, had the testimony of Edna herself that she was the speakeasy arsonist. The evidence was more than enough cause for a police officer to investigate. Due to Marty's actions, the future was eventually saved and even changed for the better.