I'm on a share plan, too, and here's how it works:
You pay for minutes once, and that covers every phone on the plan for talk minutes. The same may apply for text if your plan is a talk/text plan (mine is shared minutes only, no shared text).
Every other feature (insurance, data, text) is purchased at full price on a phone by phone basis. This means that if you have two phones that require a data plan, you are currently paying $30 per phone for data ($60/mo total). If you were to take your BB off your plan and replace it with the Thunderbolt, your data price on that phone would still be $30, and you would still pay $30 for the other phone's data. The difference would be that the $30 you pay for the Thunderbolt would include 3g and 4g, while the $30 you pay for the other phone would just be for 3g.
Bottom line: your price won't change, but you will have 4g on the Thunderbolt.
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