I'm in the process of upgrading Exchange from 2010 to 2013. The 2010 server is used with Lync 2013 as a unified messaging server. Co-existence is configured and proxy and redirects for normal exchange operations are working as expected, but not UM requests. When it hits the server Exchange 2013 appears to receive the request but doesn't pass it along to the 2010 server where the users mailbox resides.
When Exchange 2013 Servers are added to Dial Plans, users with mailboxes on the 2010 UM server can't receive voice mail, but migrated users on 2013 work as expected. When Exchange 2013 servers are not part of the dial plan than 2010 users work fine (but of course 2013 users will not).
Unfortunately a straight cut over of all users to 2013 is not possible, and I would like to ensure continued functionality for all users during the migration (otherwise I would prefer that and not worry about T/S this).
Nothing else is changing other than the Exchange server upgrade. I've validated the Dial plans, gateways, and hunt groups are all available on the 2013 server. I've followed the MS checklist here to double check my steps:
https://technet.microsoft.com/en-us/library/dn169226%28v=exchg.150%29.aspx
I'm currently waiting on a MS support rep to schedule an outage and work thru it but I'm open to any ideas.
Best Answer
Coming back to clean this up. The problem was with the certificates. For Unified Messaging the server names must be present on the subject name of the certificate. I had SAN certs, the server names were listed in the alternative names. Once the certs were replaced, we re-ran the UM scripts for good measure, and voice was working as expected.