When you say putting a new phone on the acct I think you mean adding a line right? That's what I've seen in the past but it doesn't change if you replace an existing phone while keeping the same line active. I'm hoping they stay this way so I can pick up the TB and have unlimited 4 G until 5 G comes out.
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Replacing a phone (even if you bring your own or buy at full retail) can absolutely force you off a plan. This happened with both html dumbphones and smartphones. When the Voyager was introduced, most people had MOU data plans, but this phone (and similar releases) forced users to buy a data plan. Same with smartphones in late 2007, users used to be able to have a data block, but users were then forced to buy data plans with new phones.