No service / registration failure: NAS reject causes (5GMM cause codes)

Authentication and Identity Failures

When a UE cannot register on the 5G network, the AMF sends a Registration Reject message containing a 5GMM cause code identifying the specific reason. Cause #3 "Illegal UE" indicates authentication failure — the AUSF/UDM could not verify the subscriber identity because the SUPI/SUCI does not match any valid subscription or the authentication keys (K, OPc) in the USIM do not match the UDM database. Check subscriber provisioning and SIM key synchronization. Cause #5 "IMEI not accepted" means the AMF queried the Equipment Identity Register (EIR) and the device is blacklisted, commonly because it…