I had two phone pre-interviews last year (I'm a first year in a research-oriented Clinical Ph.D. program now, which was one of the interviews), and they were, in my opinion, more nerve-wracking than the in-person interviews because it's harder to read social cues, assess conversational turn-taking, etc...
What helped me a lot was to actually type out answers to a list of questions that I thought were likely I would be asked (this is in addition to having read the POI's research in some detail, of course). I wanted to have well-formulated responses to major questions about how I fit in to their research program, overall department, etc... (probably about 40 questions in all). I also typed out questions I had for her. Maybe excessive, but better safe than sorry🙂 I didn't read off of the list, but having to answer the questions in that detail helped me organize my thoughts, and I had it there for reference in case I needed to jog my memory about something. I'm happy to email you my list of potential questions if you want to PM me. Good luck!!
p.s. I wore sweatpants🙂