Microsoft calls SIP trunking Direct SIP Connectivity, as shown in the illustration below.
VX supports direct trunking to Exchange Unified Messaging (EUM). To configure SIP Trunking to EUM, you must:
Inbound calls to Exchange UM are redirected to offload ports to improve EUM performance. The default transport is TCP and the port number is 5060. TCP callers are redirected to port 5065/5067. TLS callers are redirected to port 5066/5068. An example of a redirected call is shown below.
In the call illustrated above the call flow is:
Caller = 202-576-2345, Called = 410-572-3879, no diversion yet
Voicemail support also requires support for Diversion in SIP headers so calls not answered can be diverted to voicemail.
A user is typically configured to use voicemail for Call Forwarding No Answer (CFNA) and Call Forwarding on Busy (CFB) conditions. When an inbound call arrives to this "called" user, and they don't answer, the call is forwarded to, and answered by, their personal voicemail greeting.
The phone system (call agent/PBX/IP-PBX) sends the caller to the voicemail system and the user information in the Diversion field is used to open the right mailbox and greeting. This information is called the "Redirecting Number" or RDN (from the SS7/AIN world)
The diagram below illustrates how an unanswered call is diverted to voicemail.
In the call illustrated in the digram above the call flow is:Caller = 202-576-2345, Called = 410-572-3879, no diversion yet
When a Call Agent sees a call go unanswered, it sends an invite to the EUM pilot number and includes a SIP Diversion header identifying the original called number.