Congrats! I also had a phone conversation with my top-choice POI and it went very well.
Waiting for the call was probably the most anxiety-provoking thing of the whole ordeal. Fortunately, we didn't realize the time zone differences and called me an hour sooner than expected. Surprising, yes, but the chronic anxiety wasn't eating away at my confidence.
It was like a very informal interview. He would ask a question, I would answer, and it would stimulate some conversation. I talked about my research interests and he would discuss future directions in his lab. I also had opportunities to ask him questions (don't worry about asking about everything you want to know; if a question comes to mind after the conversation then you can always email them). The call lasted ~20 minutes and at the end he explicitly said to me that he wanted to interact in the future. If you've seen my past posts on Gradcafe, I did keep in touch and got to spend the summer as a research volunteer in his lab.
Overall, just be open (but professional), have a couple of questions ready and breathe.