For users of smartphones powered by MediaTek (MTK) processors, few errors are as frustrating as the dreaded "Invalid IMEI," "Null IMEI," or "Not Registered on Network." The IMEI (International Mobile Equipment Identity) is a unique 15-digit code that identifies your device to cellular networks. When this data is corrupted or lost—often after a failed custom ROM flash, a firmware update glitch, or a motherboard swap—your phone becomes a Wi-Fi-only tablet.