The responding node doesn't recognize this IMSI or IMEI at all (IMSI/IMEI not known).
Where you see it
In the Cause IE of a GTPv2-C response (Create Session, Modify Bearer, Delete Session…) on S11, S5 or S8.
What to check
Confirm with the home network that the IMSI/IMEI is actually provisioned.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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