Well its been a few weeks of troubleshooting and help from Microsoft PSS but I think I've finally figured out how UM handles calls on both DTMF and voice activated auto attendants.
Our client's setup is a MITEL 3300 which is connected to both OCS and Exchange UM using two distinct paths. All inbound calls are supposed to route to the Exchange server which has a simple auto attendant set up to direct calls off to extensions local and remote to the MITEL as well as OCS. Since we were concerned about a single T1 between the voice gateway and the MITEL being used up for both OCS calls and auto attendant calls, a SIP trunk was created.
We quickly had an issue with voice activated dialing where you would say the person's name but the call would go directly to voice mail. A SIP trace from Exchange UM revealed ZERO packets leaving the server. This was quite odd since keying in the DTMF codes for the extension worked fine. The calls to Microsoft were originally met with "you can't call between dial plans" to which I responded......"um, WHAT???". If you could imagine a fully deployed UM environment with multiple geographic locations all with different dial plans (for language purposes) on the same server......it is possible......read on....
The fix was simple but finding out how UM initiates a voice activated call wasn't. Since a SIP trace wasn't possible, it was up to Microsoft to provide some insight. Even with the help from Microsoft it wasn't until we dug around testing various accounts that I noticed some of them produced different results.
After opening my own test account I noticed the General tab of my account properties didn't have a "telephone" number listed.....it was blank. I entered my extension number and it worked the first time! I had made a grave mistake assuming the UM server was pulling my TEL URI field from OCS on the name match when I used the auto attendant. As a matter of fact the UM server explicitly uses this field for calling the user.
Now that we have that one fixed we're off to deploying phones and finishing the call tree for UM!