Default [Q] [Help] Switching Phones

My current phone is a Samsung Galaxy Exhibit T599 running on a 2 year contract with T-Mobile. I'm planning to buy a used iPhone 4s off of Amazon which is locked with Verizon. My dad was wondering if I would get charged any extra if I were to unlock the iPhone 4s and put my SIM card into it such as having to pay an extra $20 month that an iPhone user has to pay. Or should the phone work perfectly fine with no extra charges if I were to only put my SIM card in the iPhone. A quick reply would be nice, thanks.