Should I use in-app purchases in this case?

Should I use in-app purchases in this case?

I want to develop application for some mobile network operators, that
allow to set music for ringback tone. In details, I download available
melodies list from operator, then when user selects necessary tune I call
operator API to set it for RBT. Pretty simple.
But I'm worried about one thing - user should be charged for this service.
Off course I'll notify user about it and clearly describe amount of
charge. Operator already has billing for this service so I can use it.
As I read in IAP guides, In-app purchases should be used only for digital
goods that will be used inside application ("Items can only be used in the
app where the purchase is made"). So here is my question, does it mean
that in my case I can use operator billing and I will not have problems
with application review?