Here is the scenario:
2 Exchange 2013 servers both with CAS and MBX roles
IP PBX (UM IP Gateway) = Mitel 3300 ICP
One Exchange CAS configured on Mitel as SIP Peer
Reroute on Mitel configured when not answered or busy
Dial Plan = Telephone Extension (obviously not associated with either Exchange servers since only SIP URI can be associated with an Exchange 2013 UM)
When calling a busy or non answered extension UM answers properly and focuses on the appropriate mailbox regardless of which server is hosting the users home db. When dialing the Outlook Voice Access number from any extension configured as a UM extension on a mailbox UM properly focuses on the appropriate mailbox and asks for PIN.
PROBLEM - When calling the Outlook Voice Access number from any phone extension or outside line that is not associated with a specific mailbox, the system prompts for the extension of the mailbox you want to access. If the mailbox is in a db being hosted by the answering CAS role call router it finds the mailbox and prompts for PIN. If the mailbox is actively hosted on a db running on the other MBX server from the answering CAS router, after entering the extension for the mailbox you want to logon to, the call seems to get automatically placed on hold (perhaps even routed back to Mitel) the Outlook voice Access number called again, UM answers and prompts again. Basically the call ends up in a loop.
I tested this by moving the mounted copy of the database the user mailbox is in back to the CAS role server to which the Mitel system is configured to route calls to and all works as it should. It is only when the mailbox is on a db mounted on the other server that the calls go into a loop.