EMM cause (LTE NAS)3GPP TS 24.301 §9.9.3.9 and Annex A
What it means
IMEI not accepted: the network refused the device's IMEI (used for emergency attach without a SIM, or EIR checks).
Where you see it
In LTE NAS Attach reject, Tracking area update reject, Service reject or Detach request.
What to check
Check the IMEI against the EIR.
Look at the S6a (MME↔HSS) and S11 messages just before the reject: they usually show the real trigger.
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