Content-Type: application/sdp
Content-Length: ...* SDP goes here *Notification for Waiting Messages
Traditional analog and PBX phones have lamps that signal when there are
voice messages to be retrieved. Users content to have a similar experience with
SIP-based systems will require only a minimum of information when alerted
by a message-waiting signal.
The UA (typically a SIP phone, a PC client, or other type of device) will use
the SIPSUBSCRIBEmethod to receive NOTIFYmessages for changes of state
in the mailbox. The UA also can explicitly fetch the status of the mailbox.
The UA can subscribe to multiple mailboxes distinguished by the URIs in
the Toheaders.
Multiple UAs can subscribe to the same account. This allows the use of sev-
eral devices to retrieve waiting messages (for example, using a SIP desktop
phone or PC in the office, or a laptop, PDA or mobile phone while traveling).
We will illustrate in the following example the message notification using
SIP UAs. As shown in Figure 12.2, message waiting notification can also be
achieved through e-mail as an option, but this is not illustrated here since it is
trivial, though very useful.
Simple Message Notification Format
The simple message waiting format [3] can convey only summary information
about the status of the mailbox:
■■ Media type: e-mail, IM, voicemail, fax, and video mail
■■ Message status: new/old mail
■■ Urgent/normal messagesFor example, the message-waiting summary Voicemail: 1/3 (0/1)con-
veys the information that there is one new message, three old messages, of
which zero new messages are urgent and one old message was urgent. If no
such details are required, the message-waiting summary could simply be
Messages-Waiting: yes.
Figure 12.4 shows the message exchange required to subscribe the UA to the
mailbox and the notification messages using SIP events [7].
Voicemail and Universal Messaging 217