IMS VoNR Registration Failure — Debugging a SIP 401 Loop

Users report that VoNR calls don't work on a specific device model. Everything below IMS looks healthy: the UE attaches to 5G successfully, the IMS APN PDU session comes up, P-CSCF discovery works — but the UE never completes IMS registration. The SIP trace between the UE (behind CPE NAT, reaching the IMS via P-CSCF 10.45.1.5), the I-CSCF (10.45.1.6), the S-CSCF (10.45.1.7) and the HSS (10.45.0.4) has been captured. The UE IMPI is `user1@ims.operator.com` and its IMPU is `tel:+4930901234567` (sip:+4930901234567@ims.operator.com). The SIP REGISTER keeps bouncing back with 401 Unauthorized — the UE responds with a new Authorization header each time, but the S-CSCF rejects it again and again until, after three retries, it gives up with 403 Forbidden. The UE then de-registers and falls back to EPS/CSFB for voice. Your job: open the trace, walk through the IMS AKA challenge/response exchange (TS 33.203, RFC 3310, RFC 4169), understand the Cx MAR/MAA between S-CSCF and HSS (TS 29.229), and pin down exactly why the challenge/response loop is broken. Is this a UE fix, an HSS fix, or an S-CSCF fix?