It becomes clear that Holden is able to fully immerse himself in terms of authentic interpersonal connection with Phoebe. She is the anomaly. She represents the unbalanced part of Holden's equation regarding how the world is constructed. He overcomes his alienation with her because she is the only one that he allows in being both a part of her world and enabling her to be his only world.
The rules in which Holden appropriates reality do not apply when it comes to Phoebe. It is fitting that he sneaks into his own family's apartment to see Phoebe. She represents the purity and innocence that Holden takes as an almost antidote to the venom that he spits at the world. He overcomes his alienation with Phoebe because he wants to be included in her world. He seeks validation for his voice with her. He is visibly shaken when she criticizes him and he cannot dismiss her as he does with the rest of the world. It is here in which his alienation is only overcome with Phoebe.
The best example of this would be when Holden is legitimately surprised that Phoebe demands to come with him to Colorado. Phoebe approaches him with bags packed, ready to go. Holden cannot appropriate this into his schematic. Her authenticity and honesty are elements that do not fit into the alienating manner in which Holden sees reality. He overcomes his alienation only with Phoebe, as seen when he volunteers to take her to the zoo and watches her. He talks to Phoebe about his dream in being "the catcher in the rye." The only time that the reader sees Holden immersed in something that resembles hopes, dreams, and possibilities is with Phoebe. When Phoebe becomes the force through Holden can see what can be as opposed to what is, it becomes clear that he can only overcome his alienation with Phoebe. The element of his consciousness that rejects social interaction goes away when he is with Phoebe. It is here in which it becomes evident that Holden overcomes his alienation through his love and adoration for Phoebe.