My policy is this:
Call only once a month or so (if at all) if you still have not heard anything after filling out a secondary except in the following circumstances:
>It it 2 weeks after the date by which they have told you that they would respond to you,
>You are making a change or following up on new information about your application (e.g., change of address, grade report, additional letter of recommendation, etc.)
>You have a valid question to ask them (not "When are you going to get back to me" or questions of the like)
I know for a fact that some schools actually make a note in your application when you call. Personally, I think it is good to look interested (i.e., checking every month or so) and bad to look desperate.