Q. If the HSS returns DIAMETER_ERROR_USER_UNKNOWN in UAA, what SIP response does the I-CSCF send to the UE?
a) 480 Temporarily Unavailable
b) 403 Forbidden
c) 404 Not Found
d) 200 OK
Check 🔎
Correct answer: Letter c.
When the HSS replies to the I-CSCF with DIAMETER_ERROR_USER_UNKNOWN in response to a User-Authorization-Request (UAR), it indicates that the public identity (IMPU) is not known to the IMS system.
In this case, the I-CSCF responds to the UE with a SIP 404 Not Found.
- This maps directly from the Diameter error to SIP semantics.
- It tells the UE that the requested user identity simply does not exist in the IMS domain.
Q. If the HSS returns DIAMETER_ERROR_USER_UNKNOWN in UAA, what SIP response does the I-CSCF send to the UE?
a) 
480 Temporarily Unavailable
→ Incorrect. This implies a temporary network failure, not a non-existent user.
b) 403 Forbidden
→ Incorrect. This indicates unauthorized access, not that the user doesn’t exist.
c) 
404 Not Found
→ Correct. 404 is returned when the IMPU is unknown in the HSS, as per DIAMETER_ERROR_USER_UNKNOWN. Unknown user leads to SIP 404 to the UE.
d) 200 OK
→ Incorrect. This is a successful response, which is not valid when the user is unknown.
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