I'm pretty sure they add up the actual data for each, then round up to the nearest .1gb.. My current charges show. .4gb used, and I have three lines. One is at 215mb, another at 98mb, and the last at 40mb, as shown in the phones. That adds up to 353mb, and rounding to the nearest tenth of a gigabyte after that is .4gb. If they were rounding each phone separately, then adding that up, I'd see .5gb instead of .4gb (.3 for my first phone, .1 for the second, and another .1 for the last one).
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