Simon emails Blue and says he had a weird and awful Christmas in which he came out to his family and will soon be out to everyone else. He asks Blue to distract him from his woes and suggests that they could exchange phone numbers. In Blue's reply he tells Simon he's cute, mentions that he daydreams about Simon, and expresses concern about texting. He refuses to elaborate on his daydreams.
Exchanging phone numbers would represent the next level of connection and trust, though it would also give both Simon and Blue a little more control over the other's life. Blue's unwillingness to exchange numbers suggests he's very attuned to the ways that he could easily lose control over his story.