You have 2 options for mobile phone service in the United States: pre-paid service or contract-based service. It is recommended that you do some research before choosing a service type, selecting a carrier, and signing a contract. 

Pre-paid service

With pre-paid service, you buy a phone and add minutes to your account. This may be a more expensive option, but it gives you more flexibility because pre-paid service doesn't require a social security number or a US credit history, and it allows for short-term cell phone use.

Contract-based service

Contract-based service is sometimes less expensive than pre-paid service, but typically requires a 1-2-year commitment. Also, if you don't have a social security number, most companies will make you pay a security deposit (usually several hundred dollars). Even if you have a social security number, you might still have to pay a deposit if you haven't been in the US long enough to establish a US credit history. 

Carriers

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