I had this exact question a few months ago and could not find anything online that was helpful LOL
Basically, it was a very informal conversation where I was asked to share a little more about myself/my interests/ how I found my path to psychology and why I want to pursue a graduate degree in the field. Then we talked about the PI's research projects, and I was able to ask him whatever questions I wanted to (policies about authorship, funding, mentorship, what kinds of projects I would be helping him on, etc.). I think the point of the call would just be to find out how you would fit in their lab if you were to join it (including current projects, future projects, opportunities for growth) and what kind of mentor they are. It was very casual and laid back. I want to stress that it will more likely than not take on a very conversational tone because I did not know what to expect at all and so I was freaking out a little bit. So don't get too nervous - but still read some of their work so you can ask intelligent questions if you have any just from the standpoint of showing curiosity and interest in learning.
I did not dress up but that is because I was calling from work and at my job we dress very casually. Although if I had the chance to go back and do it again I still wouldn't dress up lol, I was just wearing a nice shirt. To further the point of not needing to be wearing a suit-and-tie, I once had a skype interview for a lab manager position and the PI was wearing a pajama tank top so.... you do not need to be all decked out in your best professional clothing.