The call forward always should be the Speed Call of the cell phone, not the cell phone number itself.
Another test I would try is set your phone (or a spare test phone) with the same COS/COR as the voicemail ports, then try and call the speed call/cell phone. See if there is an error on the display.
There are other things that could be causing this as well, like incorrect CLI being sent on the trunk. You would need to get some captures to confirm.