First, validate that the device is compatible with the UScellular Network. Then, verify that your compatible device is unlocked by contacting your current carrier before cancelling your existing service. UScellular is unable to unlock devices from other carriers and your device must be unlocked prior to activation on the UScellular network. If your device displays a message like "SIM Not Valid" or "SIM Not Recognized," when a U.S Cellular SIM is installed into it, then your device is locked and it must be unlocked by the carrier that locked it.