Cause	Name	Category	Interpretation
1	Unallocated number	normal	Number is not assigned. Verify dial plan.
2	No route to network	normal	Specified transit network unreachable.
3	No route to destination	normal	Call cannot be routed — check trunk config.
6	Channel unacceptable	normal	Channel not acceptable to sending entity.
7	Call awarded	normal	Call awarded and delivered on established channel.
16	Normal clearing	normal	Standard hangup.
17	User busy	normal	Called number is busy.
18	No user responding	normal	Called UA didn't respond in time.
19	No answer from user	normal	Ringing but never picked up.
20	Subscriber absent	normal	Subscriber registered absent (mobile).
21	Call rejected	normal	Called party rejected the call.
22	Number changed	normal	Number has been reassigned — update records.
26	Non-selected user clearing	normal	Call cleared by non-selected user (call waiting).
27	Destination out of order	normal	Destination signaling is down.
28	Invalid number format	normal	Number incomplete or invalid format.
29	Facility rejected	normal	Requested facility unavailable.
30	Response to STATUS ENQUIRY	normal	Informational.
31	Normal, unspecified	normal	Catch-all for normal clearing — usually means upstream cleared without a specific cause.
34	No circuit/channel available	resource	All circuits busy — trunk exhaustion.
38	Network out of order	resource	Long-term network outage.
41	Temporary failure	resource	Transient carrier or switch failure.
42	Switching equipment congestion	resource	Switch is congested — retry.
43	Access info discarded	resource	Network could not deliver access info.
44	Requested channel not available	resource	Signaled channel is not free.
47	Resource unavailable	resource	Unspecified resource issue.
55	Incoming calls barred within CUG	service	Closed User Group restriction.
57	Bearer capability not authorized	service	Service subscription issue.
58	Bearer capability not presently available	service	Service temporarily unavailable.
63	Service or option not available, unspecified	service	Generic service unavailable.
65	Bearer capability not implemented	unsupported	Network does not implement requested bearer.
66	Channel type not implemented	unsupported	Channel type unsupported.
69	Requested facility not implemented	unsupported	Facility not implemented.
70	Only restricted digital info available	unsupported	Only restricted bearer available.
79	Service not implemented	unsupported	Service not implemented at all.
81	Invalid call reference	procedure	Call reference unknown.
82	Identified channel does not exist	procedure	Identified channel does not exist.
87	User not member of CUG	procedure	Closed user group rejection.
88	Incompatible destination	procedure	Destination cannot handle the bearer.
91	Invalid transit network selection	procedure	Transit network value invalid.
95	Invalid message, unspecified	procedure	Generic protocol violation.
96	Mandatory information element missing	procedure	Missing IE.
97	Message type non-existent	procedure	Unknown message type.
98	Message not compatible with state	procedure	Message received in wrong state.
99	Information element non-existent	procedure	Unknown IE.
100	Invalid information element contents	procedure	IE contents bad.
101	Message not compatible with state	procedure	Protocol state mismatch.
102	Recovery on timer expiry	procedure	Timer expired.
111	Protocol error, unspecified	procedure	Generic protocol error.
127	Interworking, unspecified	interwork	Interworking issue between networks.

Tip: Cause 16 is a clean hangup. Cause 17 is a busy. Cause 31 is the "catch-all" you see when the upstream carrier gave you no detail — treat as "unknown, investigate carrier side."