Would a Regular SIM Card Work in the iPhone?by John Lister
Every iPhone contains a slot for a SIM card. This card is an integrated circuit that links the handset to a user's account with a service provider, along with the phone number linked to that account. iPhones that run on AT&T will need a SIM card to connect to the phone network, but iPhones running on Sprint or Verizon do not need a SIM card to operate. In theory, SIM cards are interchangeable but there are actually several physical and technical limitations on this ability.
Many iPhones are "locked" to a particular network, meaning they cannot work with a SIM card provided by a rival network. Locking is normal in any iPhone sold at a subsidized price by the network and accompanied by a service plan. Apple does sell unlocked iPhones at full, unsubsidized prices. Whether you can "unlock" a handset, when you can do so, and how much if anything it will cost you, depends on both your network and your contract. In a surprise exception to this usual practice, iPhone 5 handsets shipped to customers by Verizon are unlocked. This means these handsets will run on AT&T if you insert an AT&T Sim card. Taking advantage of this feature does not override any minimum contract period on a Verizon deal: you must still pay its monthly charge while under contract, even if you use a different network. To avoid this monthly charge, you can cancel your contract, but you may be charged an early termination fee.
Even if you have an unlocked iPhone, you will not necessarily be able to use it with any SIM card. This is because different handsets use one of two main technologies for voice calls: GSM or CDMA. Many networks offer service using only one of these technologies: Verizon and Sprint use CDMA while AT&T and T-Mobile USA use GSM. Your iPhone will not be able to get a voice connection on a network using the "wrong" technology, regardless of the SIM you use. Networks generally use compatible technologies for 3G and 4G data transmission. This means, for example, that a Verizon iPhone 5 with an AT&T SIM can access AT&T's high-speed 4G services.
Different models of iPhone use different sized SIM cards. Those up to and including the 3GS take a traditional sized SIM card as used in most mobile phones. The 4 and 4S use a smaller micro-SIM card while the 5 uses a smaller and thinner nano-SIM card. It is possible to cut down a full-sized SIM card to use in an iPhone 4 or 4S by removing part of the SIM card that doesn't contain any data. Some users have reported successfully cutting down a standard SIM card to use in an iPhone 5, though results may be variable because of the nano-card's slimmer frame.
Before using a different network's SIM card in your iPhone, consider how and where you will use it. Your choice of network may affect whether you can get coverage abroad and at what speeds. It may also affect whether you can use voice and data services simultaneously, for example being able to check a website while making a phone call.