A SIP Response Code for Unwanted Calls

In many countries, an increasing number of calls are unwanted , as they might be fraudulent, illegal telemarketing or the receiving party does not want to be disturbed by, say, surveys or solicitation by charities. Carriers and other service providers may want to help their subscribers avoid receiving such calls, using a variety of global or user-specific filtering algorithms. One input into such algorithms is user feedback. User feedback may be offered through smartphone apps, APIs or within the context of a SIP-initiated call. This document addresses only the last mode, where the called party either rejects the SIP INVITE request as unwanted or terminates the call with a BYE request after answering the call. To allow the called party to express that the call was unwanted, this document defines the 666 (Unwanted) response code.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 .

None of the existing 4xx, 5xx or 6xx response codes allow the called party to convey that they not only reject this call, e.g., using 480 (Temporarily Unavailable), 486 (Busy Here), 600 (Busy Everywhere), 603 (Decline) or 606 (Not Acceptable), but that the caller is unwanted. The particular response code number was chosen to reflect the distaste felt by many upon receiving such calls.

The SIP entities receiving this response code are not obligated to take any particular action. The service provider delivering calls to the user issuing the response MAY, for example, add the calling party to a personal blacklist, or MAY use the information as input when computing the likelihood that the calling party is placing unwanted calls ("crowd sourcing"). The response code MAY also be used in Reason header fields , typically when the UAS issues a BYE request terminating an incoming call.

This document register a new SIP response code. This response code is defined by the following information, which is to be added to the method and response-code sub-registry under http://www.iana.org/assignments/sip-parameters. 666 Unwanted [this RFC]

If the calling party number is spoofed, users may report the number as placing unwanted calls, possibly leading to the blocking of calls from the legitimate user of the number in addition to the unwanted caller. Thus, it is RECOMMENDED that the response code is used for creating call filters only if the calling party number has been authenticated using .

TBD.