A SIP response code meaning the called party or network declined the call, used by providers to signal that a call was blocked as suspected unwanted traffic.
603 Decline is one of the global failure responses defined in RFC 3261. It indicates that the call was deliberately refused rather than failing for a technical reason. Related codes include 607 Unwanted, which signals that the called party marked the call as unwanted, and 608 Rejected, defined in RFC 8688, which signals that an intermediary such as an analytics service rejected the call.
FCC rules require providers that block calls based on analytics to notify the caller immediately, and the approved way to do so in SIP networks is to return one of these codes, with ISUP cause code 21 used for TDM networks. The response must be passed back through every intermediate provider without being changed.
The industry has also developed an enhanced convention, often called 603+, that carries additional information so callers can learn who blocked the call and how to request a review. Whatever variant is used, intermediate providers should forward the response unchanged so that the information reaches the originating caller intact.
Why it matters to carriers
Returning the correct response code when you block, and passing it through unaltered when you are an intermediate provider, is part of staying within the FCC blocking safe harbors. It also gives legitimate callers a way to discover and fix mistaken blocking, which reduces complaints and protects your relationships with enterprise customers.