ESM cause (LTE NAS)3GPP TS 24.301 §9.9.4.4 and Annex B
What it means
Insufficient resources: the network couldn't allocate resources for the PDN connection or bearer (often PGW/SGW capacity).
Where you see it
In LTE NAS PDN connectivity reject, bearer resource reject or EPS bearer deactivation.
What to check
Check SGW/PGW load and the GTPv2 cause in the Create Session Response.
Check the GTPv2 Create Session Response and the HSS APN configuration for the same APN.
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