If it is a contracted 2 year cell phone service with T-Mobile, you basically purchase the cell phone at Walmart but your bills will be mailed to your home address from T-Mobile. You can pay your bill through the mail or you can pay at any T-Mobile store.
Walmart does have a prepaid service called Walmart Prepaid Family with T-Mobile basically it is a prepaid cell phone service without any contract but you would have to pay your bill at the Walmart store only.