The phone pre-interview seems to me like it's more to showcase your personality at a glance, get a feel for how you think on your feet, and demonstrate how you explain your reasoning, so it's difficult to prepare for - there are a few more common questions for interviews related to self-awareness and self-reflection and how you interact with others, but there are a bunch of pretty nonstandard questions, so without having specific questions (and they ask different people different things, though some of the prompts get reused year to year), I wouldn't sweat preparing too much. I won't give you the questions they asked me, as they specifically say to not do that, but I do recommend you to a) be self-aware, b) be aware of developments in biomedicine outside of what you immediately work on, c) have some fun with the oddball questions, and d) be friendly and nice. Like with any interview, you're allowed to ask for a moment to think your answers through, and they encourage you to have pen and paper to coordinate that. The phone interviewers don't see the rest of your app, and both times I did it (this cycle and the last), they were really nice - it's not pulling teeth at all.
I will say they aren't going to ask anything along the lines of "why medicine?" or interrogate your desire to become a physician in the pre-interview - that's not its purpose, and it gets covered in every other part of the application anyway.