you have to go online to buy a unlock code for the phone, once you have the unlock code you can put the sim card in and it will ask you for the code. if not you can buy a sim card holder that can...
yes, you can. as long as your sim card will fit in the sim card slot.
yes
what is a centennial phone??
no they have to be phones strictly made for at&t and the sim cards could be shaped differently and get stuck.