There maybe two reasons for this:
1.) They are overloaded with work
2.) They have your application in a pile, but will wait until they receive more before deciding if they want to call you for a interview.
UPENN is one of those schools that if you apply really really early, they will contact you really really early as well. I applied the second day and sent in my supplementary July 20 and got my interview invite August 20 and interviewed there September 20.
I would not annoy them, but just wait until the end of October before picking up the phone.
DesiDentist