I ran into an issue in applying Exchange 2010 Service Pack 3 on an Unified Messaging server - the Prerequisite Check came back with the following error:
Unified Messaging Role Prerequisites
The Unified Messaging voice mail folder 'C:\Program Files\Microsoft\Exchange Server\V14\unifiedmessaging\voicemail' isn't empty. This folder must be empty before upgrade can proceed.
Click here for help... http://technet.microsoft.com/en-US/library/ms.exch.err.default(EXCHG.141).aspx?v=220.127.116.11&e=ms.exch.err.Ex28883C&l=0&cl=cp
I went and looked, and indeed this folder was not empty. Looking at the dates on the voicemails, it was obvious that they were all old, and had been there for some time (the most recent was 6 months old). I believe there had been some previous delivery issues that had prevented them from being delivered to the proper mailbox. Since they were all old, I merely made a copy of them, and then removed them from this folder. Once the voicmail folder was empty, SP3 was able to continue installing without issue.