The main thing you would ask professors is about their research vision in for the next few years. From there, my advice would be to play it by ear. Some professors are more open and you can talk to them about their style of research, the average time it takes for a student Ph.D. under them, how much freedom they give their students while picking the research problem, etc, while others may not be so open.
However it would always be beneficial to talk to one or more of their students prior to the phone conversation with the professor (It'd help if you knew someone in the department and asked him/her to get information from the professor's students or introduce you to one of the professor's student. A student would hesitate (I sure would) reveal information (especially negative information) about their advisor to a stranger.).
About your second question, I am in the same boat as you . However, I would disagree with you when you say that after getting accepted, the student has an advantage over the PoI. If a large number of incoming students want to work with a professor, he would want to be very selective. Thus, even though the college may want you to join, the professor still has all aces up his sleeve.
BTW, in my case, I had contacted the PoI first and expressed interest in working with him, and he had suggested a Skype call. Thus, it is not like he contacted me. I'm not sure what to make of his response. The best option is perhaps to wait for a few more days.
Disclaimer: I have only had a phone conversation with a PoI, who was open and very nice. These are just my two pence.