The phone would have to be unlocked. The carrier may be willing to unlock it. Find out before you spend any money. If they won't, the unlock code may be available online for a price. Find out before you buy. There is no fee for switching phones. You simply pop in your sim card.
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If you buy it for full price, it does not have to be activated. If you buy it at the discounted price with a new contract or an upgrade, activation is required.
No. An unlocked phone is a GSM phone. Verizon is not a GSM network so the phone is not compatible. Moreover, Verizon will only activate Verizon-branded phones.