Overall, the interview will present as a conversation in which they will be feeling you out, and you them. Don't seem over-eager. Do seem polite. Since its a phone interview, give them a chance to finish a sentence and wait approx. 5 seconds after they finish to respond. Then, when you finish, wait approx 5 - 10 seconds for them to respond, and by doing so avoid interruptions. Avoid slang. Avoid embarrassing personal stories. Seem interested. Use conversational skills to reference things they may have said previously... "You mentioned before (this aspect of public health policy in your program), I too have an interest in (this aspect of public health policy in your program). Thank them at the end and say that you appreciate their time talking to you, and that the interview was a valuable experience, and that you look forward to working with them in the future, then ask when you can expect to hear back. Smile when you talk, it carries over the phone lines. This is a job interview. These aren't your friends, but rather gatekeepers to a world you desire.