You typically dont. You can talk with people there if you have general questions about the program that weren't answered during the interview.
But to tell them you REALLY wanna go there doesn't mean JACK. Places will tell you the same thing, that they REALLY want you. It doesn't mean JACK.
If you are calling to get an interview then you can ask the secretary the status of your application. THats all. A program director would get really really pissed if you some how talked your way onto his/her phone line just to ask if you were getting an interview there. Imagine receiving several calls a week about that while trying to get his/her job done.