I know it's too late for HPB but maybe this will be of use for someone else in the future. I've had 4 of these informal phone chats / interviews so far. For each one, I prepped extensively about the profs work (which no one asked me about) and about my work (useful).
Here's what people asked me:
Prof A: No questions for me. Just wanted my questions for him.
Prof B: (1) Tell me about the research you've been doing. (2) Are you sure you want to do this as opposed to working?
Prof C: Tell me why you want to study this.
Prof D: Tell me about yourself.
I was surprised that no one asked very much. All only asked 1 or 2 questions at most and then let me ask them questions for the rest of the time. If I had to do it again, I'd probably spend less time prepping on their work, since no one asked about it. Though it was nice to be able to throw in a reference to it here and there (for example in talking about how my own work connected to it).