So the call is ringing on both the deskphone and the remote destination, but the remote destination's voicemail is picking up, right? How long the remote destination rings is governed by the "Stop ringing this phone after..." field. If the remote destination's voicemail is picking up after ringing for a while, you will need to shorten this timer.
The other scenario you may be asking about is that when the user's phone is turned off or out of service range, that the cell phone's voicemail is picking up and you don't want it to. This timer is governed by the "Prevent this call from going straight to this phone's voicemail" timer. If it takes longer than 1.5 seconds (by default) of cell phone starting to ring for the cell phone's voicemail to pick up when it is out of range or off, then you will want to make this timer longer.
These timers are on the Remote Destination:
If you are talking about some other scenario, let us know and we will see what we can figure out for you.
