You can buy a laptop or Wi-Fi tablet and use it on Wi-Fi anywhere in the world, so why are cell phones and devices with mobile data not portable between different cellular networks in the same country?
Unlike with Wi-Fi, there are many different competing cellular network standards — both around the world and within countries. Cellular carriers also like locking you to their specific network and making it difficult to move. That’s what contracts are for.
Many phones are sold locked to a specific network. When you buy a phone from a cellular carrier, they often lock that phone to their network so you can’t take it to a competitor’s network. That’s why you’ll often need to unlock a phone before you can move it to a different cellular provider or take it to a different country and use it on a local provider instead of roaming.
Cellular carriers will generally unlock your phone for you as long as you’re no longer in a contract with them. However, unlocking a cell phone you’ve paid for without your carrier’s permission is currently a crime in the USA.
GSM vs. CDMA
Some cellular networks use the GSM (Global System for Mobile Communications) standard, while some use CDMA (Code-division multiple access). Worldwide, most cellular networks use GSM. In the USA, both GSM and CDMA are popular.
Verizon, Sprint, and other carriers that use their networks use CDMA. AT&T, T-Mobile, and other carriers that use their networks are use GSM. These are two competing standards and are not interoperable. This means you can’t simply take a phone from Verizon to T-Mobile, or from AT&T to Sprint. These carriers have incompatible phones.
CDMA is more restricted than GSM. GSM phones have SIM cards. Simply open the phone, pop out the SIM card, and pop in a new SIM card to switch carriers. (In reality, it’s more complicated thanks to phone locking and other factors here.)
CDMA phones don’t have removable modules like this. All CDMA phones ship locked to a specific network and you’d have to get both your old carrier and your new carrier to cooperate to switch phones between them. In reality, many people just consider CDMA phones eternally locked to a specific carrier.
Different cellular networks throughout the USA and the rest of the world use different frequencies. These radio frequencies have to be supported by your phone’s hardware or your phone simply can’t work on a network using those frequencies.
Many GSM phones support three or four bands of frequencies — 900/1800/1900 MHz, 850/1800/1900 MHz, or 850/900/1800/1900 MHz. These are sometimes called “world phones” because they allow easier roaming. This allows the manufacturer to produce a phone that will support all GSM networks in the world and allows their customers to travel with those phones. If your phone doesn’t support the appropriate frequencies, it won’t work on certain networks.
When it comes to newer, faster LTE networks, different frequencies are still a concern. LTE frequencies are generally known as “LTE bands.” To use a smartphone on a certain LTE network, that smartphone will have to support that LTE network’s frequency. Different models of phones are often created to work on different LTE networks around the world. However, phones are generally supporting more and more LTE networks and becoming more and more interoperable over time.
SIM Card Sizes
The SIM cards used in GSM phones come in different sizes. Newer phones use smaller SIM cards to save space and be more compact.
This isn’t a big obstacle, as the different sizes of SIM cards — full-size SIM, mini-SIM, micro-SIM, and nano-SIM are actually compatible. The only difference between them is the size of the plastic card surrounding the SIM’s chip. The actual chip is the same size between all the SIM cards. This means you can take an old SIM card and cut the plastic off until it becomes a smaller-size SIM card that fits in a modern phone. Or, you can take a smaller-size SIM card and insert it into a tray so that it becomes a larger-size SIM card that fits in an older phone.
Be aware that it’s very possible to damage your SIM card and make it not work properly by cutting it to the wrong dimensions. Your cellular carrier will often be able to cut your SIM card for you or give you a new one if you want to use an old SIM card in a new phone. Hopefully they won’t overcharge you for this service, too.
Be sure to check what types of networks, frequencies, and LTE bands your phone supports before trying to move it between networks. You may have to buy a new phone when moving between certain cellular carriers.