Edie Mcclurg In Planes, Trains & Automobiles (1987)
As she idly chats on the phone to a friend while oblivious to the angry customer, the "Car Rental Agent," as the character is credited, first comes out as overly upbeat. Nevertheless, she is ready to give it straight back to him following Neal's outburst.
That whole scene with the girl in #29 made me wince. It was hard to watch, but then so was the whole movie.
This.