I recieved a call form a POI shortly (minutes, frighteningly) after recieving an email acceptance letter, he basically wanted to brag about the program and how well I would fit, as well as gauge my interest, though the call was quite brief, and I felt like a doofus because I was stuck in a campus dining hall trying to hear and be heard.
As far as not having heard back yet, or expecting a call from a POI, every person is different. From my understanding, the decision process is very busy early on, as the early rounds of acceptances and rejections go out and towards the end, when they try to balance their class based on the yield from the early round acceptances. There usually is no department-wide ranking, and since PI's can usually only afford one student a year, it would be tougher emotionally to get an accpetance but know there's not going to be funding for you. However, should the person who has heard decline, you seem like a shoe in for the spot...
Just be patient, its not over till its over, thats what I keep telling myself. Being waitlisted is better than being rejected imho...