I've had phone interviews only for research industry jobs, so I am not sure if they will be different if you are interviewing for a school, but here is my experience: Employers typically set-up a phone interview in order to pre-screen candidates. They are less formal than in-person interviews, but I have still been asked specific science-based questions, and even math calculations!
Prepare as you normally would for an in-person interview (read up on the company/school, practice answers to questions) and definitely be able to answer the questions "tell me about yourself" and "tell me about your experience/research." If they feel you are a desirable candidate, most likely they will bring you in for an in-house interview.
I don't really like phone interviews because it's harder to read the person on the other line since you can't read their body language, visual cues, etc. They can work to your benefit, however, since you can have books, web pages, and notes in front of you just in case you freeze-up. Hope this helps!
