What is "phone phreaking" and how is this important to the story?

Phone phreaking refers to the practice of manipulating audio frequencies in older analog phone systems for the purposes of making free international or long-distance calls. In the novel, the Jade Key Quatrain alludes to a 70s hacker named John Draper/Captain Crunch. From the Quatrain, Wade learns that he must retrieve a Cap'n Crunch toy whistle from a house on planet Frobozz before he can open the Second Gate. In the 70s, John Draper discovered that the toy whistle in Cap'n Crunch cereal boxes could emit a 2600-hertz tone, the same one that could be used to interfere with in-band signaling in telephone switching systems.
Draper used the 2600-hertz tone to create what he called a "blue box" (a device that emitted the tones telephone companies used). In Ready Player One, the whistle Wade blows generates the same iconic 2600-hertz tone before it transforms into the Jade Key, allowing Wade to open the Second Gate. Later in the novel, Wade finds Halliday's egg and becomes the billionaire’s heir, gaining absolute control over the OASIS (Halliday's iconic global game interface).