EMM cause (LTE NAS)3GPP TS 24.301 §9.9.3.9 and Annex A
What it means
No suitable cells in tracking area: the phone should look for another tracking area or network. Commonly used as a 'soft' roaming reject, e.g. by steering of roaming.
Where you see it
In LTE NAS Attach reject, Tracking area update reject, Service reject or Detach request.
What to check
If unexpected, check whether a steering-of-roaming platform or a 4G roaming restriction is behind it.
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 #15 No suitable cells in tracking area? 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.