EMM cause (LTE NAS)3GPP TS 24.301 §9.9.3.9 and Annex A
What it means
IMSI unknown in HSS: the home network doesn't have this subscriber for EPS (or for the CS part of a combined attach).
Where you see it
In LTE NAS Attach reject, Tracking area update reject, Service reject or Detach request.
What to check
Check that the IMSI is provisioned in the home HSS.
Look at the S6a (MME↔HSS) and S11 messages just before the reject: they usually show the real trigger.
Got a trace with EMM cause #2 IMSI unknown in HSS? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.