Your going to get charged some excessive fees for GPRS access. Which is what you'll be doing. You can't be using voice minutes; explination is below.
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The blackberry's wont dial any normal isp over GSM (CSD is the protocol and blackberry's don't do it)
This is also the reason why when tethering to a laptop for example; you have to dial using the blackberry's GPRS... Trying to dialup a normal isp won't work.
See:
http://en.wikipedia.org/wiki/Circuit_Switched_Data for more information on CSD; though that page doesn't have anything concerning Blackberry's, so it's just general info if your curious.