Some people like to let people know who they've reached as soon as they pick up the phone. Companies and some individuals may answer the phone "You've reached the MAS Marketing Corporation" or "Hello, this is David Grant." When in doubt, a simple "Hello" or "Hello, this is David" will do. Unless someone(such as an employer) asks you to answer the phone in a particular way, choose a style that's comfortable for you and polite to others. Just avoid answering the phone in a way that may make the person on the other end feel uncomfortable or put on the spot (such as,"What"or"who is this?")