A pay-as-you-go SIM card is only usable in a contract phone that uses GSM technology and has been unlocked. To unlock a contract phone, you need the unlock code for your specific model.
Two types of cell phone technology are used by most cell phone service providers: Global System for Mobile Communication technology, or GSM, and Code Division Multiple Access, or CDMA. Cell phones that use CDMA technology cannot be unlocked. To unlock a GSM phone, the old SIM card should be replaced with the new pay-as-you-go SIM card. Some phones require that a numeric code be entered into the phone to complete the unlock process. This code is available from the original service provider or a third party.Learn More
A Verizon phone can work when using a SIM from a T-Mobile account. The model of phone and the carrier that one is switching to will determine whether moving SIM cards will work properly. Some cards are not compatible with particular services or may require special software on the phone.Full Answer >
The error message "SIM card registration failed" indicates that the phone is having trouble reading the electronic information encoded on the SIM card. This problem is often simple to solve.Full Answer >
An AT&T SIM card may be used in a T-Mobile phone if the phone is unlocked. SIM cards may be used in phones from different companies if the SIM card slot is the same size and if the phone is unlocked.Full Answer >
A 3G SIM card works in a 2G phone if the phone is compatible with the bands offered by the carrier. SIM cards are backwards-compatible.Full Answer >