I am attempting to plan a migration from Exchange Server 2007 to Exchange Server 2013, but I'm confused by the instructions at http://technet.microsoft.com/en-us/library/dn169227(v=exchg.150).aspx for migrating UM enabled users.

Does the Exchange Server 2007 UM server refer the incoming SIP connection to an Exchange Server 2013 server for migrated users or do I need to configure the PBX to send the SIP connections to Exchange Server 2013 for migrated users?

Is Exchange Server 2013 able to handle UM for mailboxes on Exchange Server 2007?  If so, is the legacy namespace required?

Any help that can be provided would be greatly appreciated, as I've been frustrated by the lack of documentation for migrating UM.

The previous version will not refer to the newer version, but the reverse is true as long as the UM server is in the same UM Dial Plan.  I'm assuming Lync EV is not involved here.

I can't remember if 2013 can refer successfully back to 2007, but I know it can refer to 2010 so I wouldn't see why it wouldn't work in your case.

If you are planning to migrate mailboxes slowly then you will be in a coexistence scenario and will have 2 options.

Option 1 - Create a new and separate 2013 UM Dial Plan for mailboxes hosted in 2013.  This will require a separate integration from your PBX then your 2007 UM Dial Plan is currently using.  The separate integration could use the same equipment as your 2007 is using, but the data sent to UM has to be different (pilot, diversion information, etc).  Only calls for your 2013 UM mailboxes would reach this UM Dial Plan.

Option 2 (if possible) - Include your 2013 server(s) in your 2007 UM Dial Plan, but route all of your PBX calls to the 2013 servers and let them determine whether to take the call or refer it to a 2007 UM server.

Option 1 would require you to UM disable the mailboxes prior to moving the mailbox.  Then UM enable the mailbox after the move is complete to a 2013 UM Mailbox Policy associated with the 2013 UM Dial Plan.

Option 2 would allow you to move mailboxes without UM disabling/enabling mailboxes prior to and after the move.

Thank you for the information.  I do appreciate it.