If I buy a Verizon phone from Walmart, how will I be billed?

I recently moved back to the states after living overseas as a military dependent, so I don't really know about US phones and plans. I plan on buying the Droid Incredible 2 and I saw that if you buy it from Walmart the phone is free. My mom, who is still stationed overseas, tried to buy it this morning but since I'm in the states with my aunt they wouldn't let her buy it because the shipping address is different from the one on her credit card. My question is if I use my grandma's credit card since it won't be charged because the phone is free how is the billing set up? Do they bill that card since it was the one used to buy the phone? If so, how do I change the credit card being billed? Or do you set up the billing once I receive the phone in the mail?