Sure, you could do simultanious ring from MOC to ring your deskphone as well. Really depends on where you voice mail or UM sits. If your voice mail system services OCS directly (which is going to be exchange 07) or if it services the desk phone directly. Other wise getting the call to go back to the voice mail system on unanswered calls could be tricky although probably not impossible you would just have to have your timers set correctly for ring no answer for voice mail. So my advice would be to do the forking on the system that controls where your voice mail goes would probably be easiest.
Presence states between OCS and CCM are not compatible but that not to say that you can control the function of the phone from MOC for DND. If you do simultatous ring from MOC the desk phone will not ring when MOC is in DND. If you do call forking from CCM the desk phone will still ring butt MOC will not if in DND prsence state. Does that make sense?
Cheers
Chris