Section: 13.11 Unknown User Identity
Content: The Unknown User Identity shall take the form of a SIP URI (see IETF RFC 3261 [26]). A SIP URI for an Unknown User Identity shall take the form "sip:user@domain". The user part shall be the string "unknown" and the domain part shall be the string "unknown.invalid". The full SIP URI for Unknown User Identity is thus: "sip:unknown@unknown.invalid" For more information on the Unknown 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], clauses 7.4.6 and 7.5.4.