I don't think there is one answer to this. Different regions, with different network physical infrastructures, will have different cost basis' for their charging structures. x14/penguin3107 are correct when they effectively say that a typical BES BB/user will transmit more data across networks than a BIS user. this might lead to networks "by default" charging more for BES because it causes more of an impact on their network, or because they think that is "fair" or because of associated roaming costs/agreements that are more likely to be invoked by "BES" users than BIS. nobody7290 just demonstrates another way of costing the usage-based model, using capped limits, i.e. you pay for MB (although why anyone would pay €15 for 30MB is beyond me - maybe this is an even less fair way of selling plans, by making one so ridiculous you don't really have a choice, and pay €25 for 250MB
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