Apple launched its first CDMA iPhone to U.S. users in early 2011, when it made the iPhone 4 available on Verizon's cellular network. With the iPhone 4S, the company released one model using a dual-mode chipset to work on Sprint and Verizon's CDMA networks and AT&T's GSM network. That means the phone can roam in other countries on either kind of network. Ideally, it would also mean foreign SIMs (subscriber identity modules) could be loaded into the phone to pay local rates while traveling abroad in GSM countries. But that's not how it works: The iPhone 4S comes locked to a specific network. To use it abroad, there are two choices: Buy an unlocked model directly from Apple (at a premium), or pay a cellular carrier for a global roaming plan.
With the flagship iPhone 5s, Apple released four models: the A1533, A1453, A1457 and A1530, along with four versions of the 5c. All four configurations support different radio bands, and Apple packs as many as possible in each version so that it has fewer different models to produce and manage.
No single carrier offers a clear-cut best option as of fall 2013. Sprint offers an unlimited data plan, Verizon now only offers limited data share plans that allow customers to share a set amount of data over multiple devices, and AT&T also forces new customers into similar limited data mobile share plans, but allows older iPhone customers to be grandfathered into the unlimited plan, provided they come into a retail store and request this option, and do not order online.
Carrier voice plans for the iPhone also vary in price and available minutes. Verizon and AT&T are both larger networks that may provide better cellular coverage, depending on your region. Sprint and Verizon's CDMA technology cannot use voice and data simultaneously, and the latest iPhone supports faster HSPA+ (also known as Evolved High-Speed Packet Access)data on AT&T's GSM network. In real-world use, data speeds vary by network usage and location -- Verizon or Sprint could potentially be faster than AT&T based on those factors. All carrier services are also developing much faster LTE networks, but those are only available in certain areas, and only the iPhone 5 can take advantage of LTE. You have to weigh a variety of factors and pick the best carrier, plan and phone for you.