See also: Preparing for Unified Messaging, Part 1
So, you set up your VOIP gateway and have figured out how to install the UM server. Let’s have a closer look on how stuff works.
How UM Voicemail works
Scenario 1: Somebody calls the extension (No. 101) on your desk while you’re at your desk
The Central Office (your telco) sends the call to your organization’s PBX. The PBX forwards the call to your extension no. 101 based on the configuration/coverage plan that is already configured on it. You answer the call. End of story.
Scenario 2: Somebody calls the extension (No. 101) on your desk while you’re out of office
The Central Office (your telco) sends the call to your organization’s PBX. The PBX forwards the call to your extension no. 101 based on the configuration/coverage plan that is already configured on it. Of course, nobody is there to answer the call, so the call is returned to the PBX.
Based on the hunt group that you have configured on the PBX, it can be made to forward the unanswered call to the Unified Messaging server. The PBX (if not IP enabled) will have the VOIP gateway behind it, which will convert the circuit switched network to a packet switched network that the UM server can understand. A non-IP PBX cannot talk directly to the UM server.
The VOIP gateway uses the SIP (Session Initiation Protocol) to establish a session with the Unified Messaging server over a packet-based network. Once the session is established by the SIP protocol, transmission of voice to the UM server is done by the RTP protocol (Realtime Transport Protocol).
The UM server queries the Active Directory to find out who’s extension is ‘101’. This information is found by querying the AD for the user account that has Office telephone number attribute set to ‘101’. Once it looks up the user, the UM server connects to the mailbox server (using MAPI) to fetch your prerecorded greeting message: “Hi, this is Shijaz and I think I’m not at my desk now, could you please leave a message?”
The UM server then records the caller’s voice message and attaches it in WMA format to a new email message and sends the message to your Hub Transport server (using SMTP). The caller hangs up. The Hub Transport server eventually delivers the email (with voice attached) to your inbox.