RFC 8688: A Session Initiation Protocol (SIP) Response Code for Rejected Calls

This document defines the 608 (Rejected) Session Initiation Protocol (SIP) response code. This response code enables calling parties to learn that an intermediary rejected their call attempt. No one will deliver, and thus answer, the call. As a 6xx code, the caller will be aware that future attempts to contact the same User Agent Server will likely fail. The initial use case driving the need for the 608 response code is when the intermediary is an analytics engine. In this case, the rejection is by a machine or other process. This contrasts with the 607 (Unwanted) SIP response code in which a human at the target User Agent Server indicates the user did not want the call. In some jurisdictions, this distinction is important. This document also defines the use of the Call-Info header field in 608 responses to enable rejected callers to contact entities that blocked their calls in error. This provides a remediation mechanism for legal callers that find their calls blocked.