I ended up going with 7 digit mailboxes. For example:
Site Codes:
San Jose - 300
San Fran - 400
New York - 500
If John Doe is at the San Fran office, I have his mailbox extension at: 400xxxx (xxxx= 4 digit extension)
In CallManager, we put in voicemail routes that would prepend the site code to the 4 digit so Unity could pick it up correctly.
If I added a John's phone at the San Fran site, we configured a voicemail profile for that phone to roll to Unity with prepending the digits.
It all works fine so far. We had to train the users and help them understand why we went from a 4 to 7 digit.
I also created a CallHandler for each site when they call in the "backdoor" to Unity to check voicemail.
So for San Fran, the subscribers would call a local exchange number to that office for instance, 415.341.1000
This number would hit the router, forward to Unity, The callhandler would listen for this number and play the greeting for that number.
"You have reached company xyz san francisco voicemail network. To login into your voicemail, press * and enter your 7 digit login id. Remember, you 7 digit login id is 400 and your 4 digit extension."
We would do this for each site to help them adjust to checking voicemails remotely when dialing into the system.
