603 6xx Global failure RFC 3261
SIP 603 Decline
The callee's device was reached but the user explicitly declined the call.
Typical Q.850 / ISDN cause: 21 (Call rejected)
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Common causes
- The user pressed decline or reject.
- Call-screening or blocklist rules.
- Spam-call protection by the carrier.
How to fix it
- Usually intentional. If you get many 603s on outbound campaigns, your caller ID may be flagged as spam. Check its reputation and attestation.
Seeing 603 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 6xx responses
The request cannot succeed anywhere. The caller should not retry it elsewhere.