From SIPfoundry sipx, The Open Source SIP PBX for Linux - Calivia

[edit] Enabling voice mail message waiting indication

If your network includes a voice mail server that can send SIP NOTIFY messages (such as the sipX Media Server that is part of sipX, the open source SIP PBX for Linux), you can configure the sipXphone in your network to receive those messages and alert users when voice mail is received or other status changes take place. There are two options for setting up this process:

• Configure the phone to send a SUBSCRIBE message to the server each time it starts up by storing the voice mail server’s location for the PHONESET_MSG_WAITING_SUBSCRIBE parameter. The server can then send SIP NOTIFY messages to the subscribing IP address. Phones with either dynamic (DHCP) or static IP addresses can use this subscription method.
• Configure the voice mail server to send SIP NOTIFY messages to specified IP addresses when a status change occurs. This method is feasible only for phones with static IP addresses.

Note: After you configure the server and phone, the phone indicates voice mail status on its home screen by displaying either No Voice Mail or New Voice Mail with the number of new messages. The phone’s message light turns on when it receives a new message.

You can also simplify user access to the voice mail server for retrieving messages by configuring the PHONESET_VOICEMAIL_RETRIEVE parameter.

PHONESET_MSG_WAITING_SUBSCRIBE

This parameter stores the address for subscribing to message waiting status indication for the sipXphone user’s voice mail box. For example:

PHONESET_MSG_WAITING_SUBSCRIBE : sip:sub-user@vm.company.com

Note: The sipXphone automatically sends a SUBSCRIBE message to the specified address each time it starts up and re-subscribes before the subscription period expires. The voice mail server subsequently sends the phone a NOTIFY message each time the message waiting status changes.

PHONESET_VOICEMAIL_RETRIEVE

If you store the retrieval address for a user’s voice mail messages with this parameter, the user can press a single button to dial the voice mail server for message retrieval. You use the browser-based interface to specify a complete SIP URL for retrieving messages from the target mailbox. Some examples follow:

sip:sub-username-retrieve-inpin@sipserver
sip:username@vm.mycomp.com;function=retrieve
sip:username@voicemail.mycomp.com;msgId=4

If you set a value for this parameter, the phone user can press the button to the right of the No Voice Mail or New Voice Mail indicator on the sipXphone home screen at any time to dial the voice mail server and retrieve or manage messages.

Tip: The New Voice Mail indicator will still display in response to a SIP NOTIFY message even if you do not set a value for this parameter. However, the user then must explicitly dial the voice mail server to retrieve or manage messages.