Section: 13.6 Anonymous User Identity
Content: The Anonymous User Identity shall take the form of a SIP URI (see IETF RFC 3261 [26]). A SIP URI for an Anonymous User Identity shall take the form "sip:user@domain". The user part shall be the string "anonymous" and the domain part shall be the string "anonymous.invalid". The full SIP URI for Anonymous User Identity is thus: "sip:anonymous@anonymous.invalid" For more information on the Anonymous User Identity and when it is used, see 3GPP TS 29.163[ Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks ]  [63].