First of all, breathe! If you're like me you will feel less anxious if prepared, so I suggest that you do the following:
1. Think of likely questions they will ask you. These include:
"Why did you choose this field?"
"Why this school/program?"
"What do you want to do with this degree/Why the PhD?"
"What are your research interests?"
"Tell us about your experiences/what makes you qualified for this?"
"What are your strengths/weaknesses?" You can also look up situational interview questions for job interviews, as some wil ask you those.
2. Run through a mock interview either with yourself in a mirror or with a friend. It can be as simple as you asking the question and then answering it back.
3. Don't be afraid to take time to answer. They understand that you need time to arrange your thoughts.
4. Have a list of things about your background/experience/whatever that you absolutely know they need to hear. Something that sets you apart, yor good/unique skills, etc. and work them into the conversation/your answers. You can physically cross them off the page (since it's a phone interview) as you go if you want.
5. Have a list of at least 10 questions you want to ask them. This is crucial, and often overlooked. I say 10 because it is likely that some will be answered over the course of the conversation without you asking them, which may cut down on the number you have at the end.
6. Relax, and smile! Nervousness will show in your voice. Smiling makes you sound more cheerful and will slow down your speech (in a good way).
7. If you're like me you will have a scrap paper to doodle "fuck!" and other expletives on when things don't go well.
Good luck!