as the call has been forwarded, the coverage to the cell's VM is out of the Avaya control. Essentially the call will foward off the PBX and ring at the cell until the caller disconnects, or the cells vm kicks in to answer the call.
if you were to setup EC500 from your desk to cell, in which case both devices would ring simultaneously, then the vm you end up in depends upon the numbers of rings each device is set to before going to vm. If your desk is set to 5 and the cell to 4, then in this case, calls would end up in the cell's vm. If the values were reversed, then the pbx vm would be the one to take the calls.
As a long time user of EC500 and now One-X Mobile, and only ever advertising my number as my desk phone, i have never had vm setup on my cell, so i only ever have one repository to check for vm.