... card?
Answers (2)
In the U.S., there are two wireless technologies — CDMA and GSM — and they are completely incompatible with each other. The majority of the Verizon and Sprint networks use CDMA technology, while AT&T and T-Mobile are built on GSM.
The big difference is that GSM networks use a SIM card to store a subscriber's identity and info, while CDMA devices have electronic serial numbers embedded in the device and require reprogramming to change networks.
With a SIM card, you can quickly swap your service provider by inserting a new card into your phone. If your T-Mobile or AT&T phone is unlocked, you should have no problems swapping SIM cards between devices on these networks.
Since the network operators launched LTE networks, it's become a little easier. All LTE networks in the U.S. are built using GSM technology, so newer Sprint and Verizon LTE-enabled handsets include a SIM card and may work on all GSM networks, big and small.
It depends on the way specific phones are designed though. Some older phones only use the SIM card for LTE services, meaning that even if you can switch to a new GSM network, you might not be able to access 2G and 3G services for calls and messaging.
The only sure way to know is to contact the carrier you wish to switch to and ask them if the phone will work on their network.
Theoretically you can move a SIM card to any GSM phone, but in fact some phones are programmed to only work with one carrier. That is called "locked". So you have to be sure the phone you want to use is unlocked. Second, SIM cards have changed, so an old SIM might not work in a new phone. And third, your phone might not work for unknown reasons and your carrier does not want to be bothered with your problems. They just want you to buy one of their phones and keep sending money. I have run into all three of these problems in the last three months.