Hi,
I definitely do not have as much experience/knowledge as a lot of people on this thread (this is my first time applying to Clinical Psyc programs and I've had 1 prelim phone interview so far). However, I figured I would share my experience since it might be helpful.
For my prelim phone interview, my POI started the interview with "this is an informal style interview... do you have any qs for us?" I'm sure this varies widely for phone interviews, but most of my phone call was based largely on my own qs, which guided the interview. I would highly recommend prepping a word doc of qs to ask during the phone interview. Your POI will likely ask if you have any qs, and it's important that you ask qs to show that you are interested in their program and have done your research. Mitch Prinstein's guide has some excellent interview qs that are useful http://mitch.web.unc.edu/files/2017/02/MitchGradSchoolAdvice.pdf
It also helps to touch up on your CV and to be able to explain your research interests and how they relate to/ have prepared you for grad school and your POI(s) interests. Have an elevator speech prepped about your research + interests and possibly look up some interview qs on Google to help yourself prepare. These are the things that helped me!