SIP response codes
SIP 603 Decline — the call-blocking response.
SIP 603 tells the caller's network that a call was deliberately refused. It — and its newer cousin 608 — are the right signals for policy-based call blocking and robocall mitigation.
Definition
What SIP 603 means
603 Decline is a global-failure response defined in RFC 3261. It indicates that the called party — or a network element acting for it — intentionally refused the call. Unlike 486 Busy Here or 480 Temporarily Unavailable, it is not a transient condition, so upstream networks should not simply retry another route.
For carriers running robocall mitigation, that clarity matters. A 603 tells upstream providers the call was refused by policy rather than failing for technical reasons, which keeps routing behavior sensible and your records clean.
Common rejection codes compared
| Code | Meaning | When to use it |
|---|---|---|
| 480 | Temporarily Unavailable | Callee not reachable right now; retry may succeed |
| 486 | Busy Here | Callee is busy at this location |
| 603 | Decline | Call intentionally refused; safe default for policy blocking |
| 607 | Unwanted | Called user marked the call as unwanted (RFC 8197) |
| 608 | Rejected | An intermediary (e.g. analytics) rejected the call; can include redress contact (RFC 8688) |
603 vs. 608
RFC 8688 introduced 608 Rejected specifically for calls blocked by network analytics rather than by the person being called. A 608 can carry a Call-Info header with a contact so the originator can learn why they were blocked and ask for a review — supporting the FCC's expectation that blocked callers have a way to seek redress.
Where a redress contact is available, 608 is the more precise choice. 603 remains a reasonable default when it is not, or when downstream equipment doesn't yet handle 608 well.
Automated handling
The right response code for every block
VoIP Radar returns the appropriate SIP response for each blocked call — 608 with a redress contact where configured, 603 as the safe default — so upstream carriers get accurate signaling and your traceback record stays clean.
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