e value id to the header Privacy in the response. If the callee (Bob or Carol) has subscribed to TIR, their telephony application server has to insert the header Privacy with the value id. If the callee (Bob or Carol) has subscribed to TIR, they can temporarily raise the restriction by indicating the value none in the header Privacy. 5.2.1.3. Message waiting indication Message waiting indication (MWI) enables the telephony application server to indicate that a communication has occurred and that a voicemail (for instance) has been recorded. In this case, the TAS entity fulfills the function of a voicemail messaging service. An MWI is transmitted if the user has subscribed to the MWI service. Alice's UA entity transmits a SUBSCRIBE request to subscribe to the MWI service. This message contains the header event with the value message-summary. It indicates in the header Accept the type of message body (value application/simple-message-summary). SUBSCRIBE sip:Alice@homeA.net SIP/2.0 Event: message-summary Accept: pplication/simple-message-summary The request is transmitted to the telephony application server by the S-CSCF entity following the iFC data processing. 160 VoLTE and ViLTE The telephony application server verifies that the subscription for Alice's identity, contained in the header P-Asserted-Identity, is authorized and responds with 200 OK. Otherwise, the response is 403 Forbidden. When the subscription has been successful, the telephony application server immediately sends a NOTIFY request to synchronize the current state of the messages waiting. This initial notification contains only brief information about the recorded messages. NOTIFY sip:Alice@homeA.net SIP/2.0 Subscription-State: active; expires=86399 Event: message-summary Content-Type: application/simple-message-summary Content-Length: (...) Messages-Waiting: yes Message-Account: sip:Alice@homeA.net Voice-Message: 2/1 (1/0) Video-Message: 0/1 (0/0) The message body of the NOTIFY request contains the following fields: - Message-Account: this field i