When Andy tosses her ringing phone into the fountain in Paris, it isn't just a rejection of Miranda; it is a reclamation of her own boundary. She realizes that being "good at her job" was turning her into someone she didn't like.
She proves that even Andy's "lumpy blue sweater" was carefully selected for her years prior by the very people she looks down upon.
(or alternatively: "Meet Andy Sachs. A million girls would kill to have her job. She's not one of them." ).
Andy doesn’t wake up one day and decide to become a cold, calculating corporate ladder-climber. It happens in microscopic shifts. First, she changes her clothes to make her job easier. Then, she sacrifices a personal dinner.