You can activate any smartphone you want on your data line without losing unlimited data. You just can't buy said phone from verizon or the other sellers for verizon (best buy, wirefly, etc...) with a subsidized price. You have to pay full price ($500-700) to keep unlimited. I think by "trying out" the other phone they returned it within the 14 day return period.
BTW, you can activate the new phone over a phone with verizon using a landline or another mobile phone. And the upside is most of the csr people know what they are doing and can be supremely helpful if you are nice with them.
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