The official answer is that the first 100mb per billing period. Not per gig. So you essentially won't get a $12 charge until you pass that 100mb. Seriously that WILL be hard to not pass. You should turn off roaming data for starters It's under the cellular tab in your phone settings. Then find roaming and set it to voice only. I use just under a gig each month so I pay for just my $12 but I still need to turn it on from time to time. They have WiFi everywhere but not "everywhere" I go.
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