I just had a first phone call with my POI. In the email, he said he would call me to answer my questions. I was like "sure." And I prepared quite a few questions.
Then today, he called and bombed me with his questions immediately. It was really tough. There were some details about my research from two years ago that I really could not recall. He kept digging, and when he heard about the research I did this summer, he even asked why my adviser would start a project like that (He seemed to find this research pointless). I totally stuttered. How the hell should I know? My adviser said it was an interesting project and I found it interesting too, so I took it. How am I supposed to know why my adviser found this project interesting? In the end, he only gave me a few minutes to answer my question and he kind of lost patience when he found out I prepared more questions than he expected.
@Radian, your comments about the professors are probably right.