SIP 603 Decline is how a network says "the callee refused this call." It's the correct response for policy-based call blocking, STIR/SHAKEN failures, and robocall mitigation actions.
SIP 603 Decline is a final response defined in RFC 3261. It signals that the callee — or a proxy acting on the callee's behalf — has explicitly refused the call. Unlike 486 Busy Here or 480 Temporarily Unavailable, a 603 says the rejection was intentional.
For carriers running robocall mitigation, 603 is the right code to send when a call is blocked by policy: the upstream provider sees an unambiguous "refused" state and knows the call did not fail for network reasons.
RFC 8688 introduced SIP 608 Rejected specifically for network-based call blocking. 608 includes a Call-Info header pointing to a redress URL so the originator can dispute the block. For pure robocall mitigation, 608 is preferred — but 603 is still the correct default when no redress endpoint exists.
SipShield emits the right SIP response code for every blocked call — 608 with a redress URL where available, 603 as the safe default — so upstream carriers get accurate signalling and your traceback record stays clean.
Robocall Mitigation