The situation you are describing is very similar (but not the same) to “blind forward”. In this case, the call is forwarded by the mediation server immediately and OCS does not participate (except in the initial state – to instruct Med. to initiate forwarding).
You could use “Simultaneous Ring” feature. Assuming MOC is set to forward to Voicemail in 20 seconds and your cell phone – in 30 seconds, if no answer from both, the call will be forwarded to Exchange Voicemail on the 20th second and the cell phone will stop ringing. If both (MOC and your cell phone are not active, the cell phone’s Voicemail will kick in immediately.
In any case with “Simultaneous Ring” the call will be captured one way or another.
Drago