Your SIM card won't work in another phone how can you make it work?

Because in the US all GSM devices (ones that require a SIM card) are SIM locked to their respective carrier (AT&T or T-mobile). Before you can use your SIM in another carrier's device you must get the SIM Unlock Code (some call it a Subsidy Unlock Code) from the service provider that the device was manufactured for and then they will also give you the instructions for inputting the code into your device.