I just don't understand how a phone call can make a difference. To me it seems that a program has evaluated your application and CV and has gotten to know you as well as they can (or care to) through the interview. After that, they decide who they want based on merit and compatibility. The applications probably contain letters from the people who would be making these phone calls.
If I were a program director, even a phone call from my best friend would not compel me to move an applicant up the list (and effectively others down). They already have all the information necessary to decide who the want and how badly they want them. This is why many programs rank right after meeting, because they don't want time to alter their evaluation of candidates.
I can see the argument for nepotism or back scratching or cronyism, but again if I am playing the role of program director, I would want the best people an not just somebody who is well-connected. I think you might be able to get an idea about whether or not this could happen at a program based on how the interview day goes and how involved everyone is. Otherwise, I agree with guttata that it would (or should) require a meeting of the ranking committee.
I am interested in hearing what others have to say about this, including personal experience (obviously I have none). I have had this discussion with friends applying to other specialties. One possible conclusion that we have reached is that in a specialty like medicine, where a program may interview and rank 300 applicants to fill 40 positions, these phone calls may be useful to move applicants around in the 30- 100 range. In smaller specialties (60 interviewees for 4 spots) it makes more sense that the ranking committee knows those 4-8 people on top well enough to make their own decision.
