Although I've never done a phone interview personally, my girlfriend went through several of them recently as part of her job search. As far as I can tell, phone interviews are primarily a screen to reduce the number of possible candidates to invite to in-person interviews. If you do well in the phone interview, expect to be asked to come into the office to talk some more.
Although phone interviews may seem to be easier because you don't have to worry about dressing up, shaking hands, getting lost on the way to the office, and so on, I would suspect that they are more challenging than an in-person interview in certain ways. A huge percentage of communication is done through non-verbal means: appearance, demeanor, gestures, facial expressions. On the phone, none of those factors can work in your favor. Even verbal clues such as inflection can be missed or misinterpreted more easily because of the phone. Finally, it may be harder to keep the attention of the interviewer -- he/she could be half listening, and half reading email, staring out the window, etc.
If I were you, I'd pay attention to speaking deliberately and clearly into the phone, keep your answers on topic and interesting, and do your best to project your personality in your voice. It may be helpful to sit and gesture while you're on the phone as if you were in a face-to-face interview to establish the right state of mind.