Here's how you can prepare for a telephone/Skype interview.
1) Prepare to speak "about yourself" for a minute or so. Talk about your academic activities after college (i.e., from the university and above).
2) Mention any significant or notable achievement. For example, top student in class, any medals, any publication, conference attendance, etc.
3) Mention something about your past and current research activities. Leave an impression that you are research-minded and you would like to continue research in area "A" or whatever is appropriate.
4) Do mention that you have interest in a particular area or areas and give a specific example of a research topic that falls under the professor's research field. Avoid being too pushy. Do not only say "I am very interested to work with you"-type statement. If you say so, say why you are interested immediately.
5) Be familiar with the professor's work, recent publications, lab environment, etc.
6) If opportunity comes, ask the most relevant question
7) When opportunity comes, mention any of the professor's work that matches directly with your previous work. Lastly, don't be nervous. Practice what you are going to say. Say it slowly and steadily but naturally.
9) After giving an answer, you might want to ask if the professor have heard your answer. You can ask something like this: "Did I answer your question?"
10) Do not ask like this: "Did you understand my answer?". The the word "understand" is not a polite word, especially in telephone conversation.