I do not get charged per kilobyte and I'm a Cingular customer. You need to keep calling. Do not let them sign you up for data connect or medianet. It is simply something that needs to be added to your account. My BlackBerry couldn't use the APN out of the box and I called 3 different lines (611, then BlackBerry support, and finally my company's dedicated support line). I finally got a rep who knew how to set up my account.
The thing is that the rep on the BlackBerry support line told me I need a data connect plan and I said "fine, put a pay per usage plan on my account." He claimed that one did not exist in the system. To me, that is a sign that it is a problem with the provisioning.
If I were you, I would email this address:
it is the office of the CEO. I think its definitely worth escilating it to this level... every other day there is a thread on some wireless forum about how Cingular screwed up their BlackBerry billing. I'm sure BlackBerry is a very important product for Cingular and the higher ups should know how the CS reps are screwing them out of money from people cancelling their BB service.