Microsoft Office Communications Server 2007
Enterprise Edition using Direct SIP to Cisco Unified
Communications Manager 7.0(1) Simultaneous Ring
Here is the guideline how to: http://www.cisco.com/en/US/solutions/collateral/ns340/ns414/ns728/ns784/716742.pdf
I don't think that Dual forking (Simultaneous ring) is a good solution, because you don't get "in a call" status in Communicator when using the phone, only when using the Communicator as endpoint you change status to "in a call".
If you use Dual forking and Remote Call Control you will have "in a call" status even when you are using the phone. it's only Nortel that supports Dual forking and Remote Call Control right now.
If you need to understand all the different way to connect to your phone system, please look at Jochen Kunert's blog: http://blogs.technet.com/jkunert/archive/2008/07/30/voice-scenarios-with-ocs-2007.aspx
Here is the Supported IP-PBXs for Microsoft Office Communications Server 2007 link :http://technet.microsoft.com/en-us/office/bb735838.aspx#ippbx2