Think this paper has what you need

Message Waiting Indicators (MWIs)

http://www.cisco.com/en/US/partner/docs/voice_ip_comm/unity/405/troubleshooting/guide/ex/tsg_0500.html

the main flow of this is after the message arrives to exchange, exchange sends a TABLE_ROW_ADDED (you can use the MBXSuite to see this) event to the subscriber, this triggers the MWI behavior in unity and then unity uses one of it's out ports to send the MWI signal (you can use the port status monitor to see this)

in port status monitor you do not see if it's to set it on or off, you only see the Dialing MWI(DN) message

it hits one of the MWI DNs in CCM, which then dials the users DN and sets MWI on or off

VM ports reach MWI DNs, MWI DNs reach user DN

you can see all this in the traces but i think the link i posted is a good explanation without the need to read thru traces

BTW, the way this integrates is via skinny, think of the VM ports as virtual phones

HTH