How to prepare for a phone interview with a prospective advisor?
If you're interested in joining the group, then you should be able to show that you've done a little bit of prep work (which clearly you have, since you've talked about his papers).

Questions I'd like prospective students to ask me include:


What projects do you have upcoming?
What is your advising style like?
What are your expectations for graduating students?
Could I talk with some of your graduate students/postdocs?