I thought I'd pass along my experience with some undocumented UM changes in Exchange 2010 SP1 that recently caused a client some grief.
This particular client has made extensive use of UM dial plans and auto attendants, with it all tied in to a Cisco Call Manager telephony environment. The way they configured their AAs wasn't done in a manner recommended by Microsoft. Specifically, they didn't consistently assign Dialing Rule Groups to their AAs.
If you're not familiar with Dialing Rule Groups, they are essentially groupings of dialing rules used to determine the types of calls that users can make when they make outgoing calls via Exchange UM. For instance, you might have a dialing rule group that contains a set of rules that only allows local calls. According to MS Best Practices, every dial plan and auto attendant should have at least one dialing rule group assigned to handle every possible combination of numbers it is expected to see.
The UM Dial Plans used by this client were almost exclusively set to 4 digits. Many of the AAs had key mappings (ie Press 1 to reach Sales) that routed to 7-digit extensions. There were no dialing rule groups in place on the AAs. In Exchange 2007 and Exchange 2010 RTM, this didn't seem to matter. The auto attendants always routed the calls properly.
However, once we put in SP1, all the auto attendants that routed calls to extensions with more than 4 digits failed. Users would get to the main menu, press the button corresponding to the key mapping and get a message saying the call could not be completed, and the caller was returned to the main menu.
After much sweating, hand-wringing and a call to MS Premier Support, we determined that we required a dialing rule group on each of the auto attendants that routed to 7-digit extensions. Once done, calls were routed as before.
The Microsoft support rep said that the AAs should never have worked using the configuration this client had in place. However, this client had successfully used this method in both Exchange 2007 and Exchange 2010 RTM. It was only Exchange 2010 SP1 where this became an issue. One way you could look at it is that Exchange 2010 SP1 corrected an logical oversight in previous versions.
So, in essense, make sure your dial plans and auto attendants are configured according to Microsoft's Best Practices BEFORE upgrading to Exchange 2010 SP1.