NAS Cause Code
Error codes in NAS reject messages indicating why a procedure failed (e.g., #5 IMEI not accepted, #11 PLMN not allowed, #27 insufficient resources).
When a NAS procedure gets rejected — a registration, an attach, a PDU session request — the reject message carries a cause code, and that number is usually the fastest route to the actual problem. They show up constantly in QXDM/QCAT logs and in core-side traces, and learning the common ones saves a lot of time.
A handful you'll meet often:
- #5 — PEI/IMEI not accepted: the device identity was rejected — "PEI not accepted" in 5G, "IMEI not accepted" in 4G — often an EIR/blacklist hit or an unprovisioned device.
- #11 — PLMN not allowed: this network isn't permitted for the subscriber (roaming agreement or provisioning).
- #27 — meaning depends on the release: in 5G (TS 24.501) it's "N1 mode not allowed"; in 4G (TS 24.301) the same number is "service option temporarily out of order." A textbook case of why you can't assume LTE and NR cause meanings line up.
The number isn't the root cause — it's the pointer to where to dig. And because the meanings can differ between 4G and 5G, look an unfamiliar value up in the right spec rather than assuming LTE and NR match.
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