IMS Registration can indicate "Not Registered" for any number of reasons -- defective or missing UICC (SIM card), corrupt radio firmware, mismatched hardware blobs due to incompatible ROM, etc. The primary cause, however, is simply a non-provisioned device & carrier. If you are using a Sprint, Boost Mobile, Virgin Mobile, TextNow, FreedomPop or Ting compatible device, and your device has been activated, open your stock dialer and dial ##72786# then select YES on the SCRTN menu. This resets your device's network, telephony and RIL settings to factory default, reconfigures your telephony.db and telephony.journal libs, prompts an update of your PRL and client-initiated profile, and initiates IMS registration of your device & carrier.