In OCS 2007 R2, the decommission steps are widely documented by both Microsoft Technet and many other bloggers. And with Lync 2010 and Lync 2013 now being generally available, I find myself cleaning up a few OCS 2007 R2 implementations. I got fairly comfortable with the steps, and I goofed on the order of things. I removed the assigned conferencing directories to an OCS pool before I deactivated the Conferencing Attendant.
I found this related post on Lee Desmond's blog, which matched the error I was receiving in my attempted deactivation:
"A call to a subtask failed.: The call to subtask AppServer.GetAppState failed. Pool is not ready."
The error associated was 0xc3FC200D
I searched a while and finally found that I made the mistake of removing the conferencing directory prior to deactivating the Conferencing Attendant. Simple, enough - I'd just need to create a new conferencing directory. Only there is not a way to do so in the OCS GUI. So I had to take to ADSIEdit.msc for this one.
Much of this is thanks to this blog entry, which solves a different issue, but contained most of the knowledge I needed!
**Thanks to Refik Unver for the Update in the comments, which has now been added to the post!