why does gatsby get so many phone calls? why does this say about him?

Gatsby'a phone calls are probably about some illegal business dealings. The reader only gets to hear Gatsby's end of the conversation, and even that doesn't clearly indicate what Gatsby is talking about if we only look at one conversation. If we put together what we know about Gatsby and his associations with people like Meyer Wolfshiem, then we can deduce that Gatsby is dealing with business matters that are not legal. At the start of chapter 5, Gatsby awkwardly attempts to repay Nick for arranging the get together with Daisy. Gatsby tells Nick that what he is proposing is a business matter where Nick "...might pick up a nice bit of money." Then he goes on to say, "It happens to be a rather confidential sort of thing." If it were legal, it probably wouldn't be confidential. This tells us that Gatsby is most likely, as Tom points out, a bootlegger of some sort. This also alludes to the idea that Gatsby was willing to resort to any means possible to earn the money he needed in order to impress Daisy and hope to get her back.