What happens to the iPhone contact when the amount is not paid?

I want to buy a iPhone 5S from USA please suggest me the methods to buy a locked iPhone and please also guide me to the point that what happens with the contract?

Are you trying to buy an iPhone at discounted contract price and then ditch on the contract? I'm confused by the question. The carrier will have all of your personal information and can bill you for the contract, if you do not pay it they WILL take you to collections and get their money. If you don't pay it, that "cheap" iphone will cost you dearly in your credit score. If you plan to buy it and just cancel the contract be aware that all carriers that offer discounted iPhones with a contract charge an Early Termination Fee or ETF for short and they are in the neighborhood of $350-400.

Walk into a store that offers it, & ask for it, exchange money & you are good to go.

You pay the contract over the course of 2 years based on the plans options you take at the time of purchase.

If it is not paid then they cancel the contract & you lose service.