Apple has started offering you the option to buy an iPhone 5S unlocked, meaning you can use it on any GSM carrier.
However, T-Mobile and AT&T are your only options for U.S. carriers that will work with the unlocked iPhone.
Since there is no contract for the device, the new iPhone 5s will cost $650 for the 16GB version, $750 for the 32GB model, or $850 for 64GB of internal storage.
You also have the option to buy thew iPhone 5s either SIM with a T-Mobile SIM, again without a contract.
If you buy iPhone for T-Mobile, it will arrive with a nano-SIM card already installed that you can activate by visiting an Apple Retail Store or a T-Mobile store or by calling T-Mobile. If you buy a SIM-free iPhone, you will need to purchase a GSM-compatible nano-SIM card separately. In the United States, you can purchase such a SIM from AT&T or T-Mobile.
When you travel internationally, you can use a nano-SIM card for iPhone 5s from a local GSM carrier.
Apple also offers the iPhone 5C, iPhone 5 and 4S unlocked.