If you use Mobile Twinning to a cell phone with the IP Office Essential Edition here is what to expect.

• If the cell phone is ON and the twinned call is not answered, the caller will go to the Voice Mail Box with the shortest time out.
• Timeout Example: Cell phone is ON
  The cell phone sends unanswered calls to its Voice Mail Box after 15 seconds of ringing.
  The IP Office sends unanswered calls to its Voice Mail Box after 10 seconds of ringing. In this case, the caller will go to the IP Office Voice Mail Box since it has the shortest timeout.
• Timeout Example: Cell phone is ON
  The cell phone sends unanswered calls to its Voice Mail Box after 10 seconds of ringing.
  The IP Office sends unanswered calls to its Voice Mail Box after 15 seconds of ringing. In this case, the caller will go to the cell phone Voice Mail Box since it has the shortest timeout.
• If the cell phone is OFF and the Mobile Answer Guard timer option in the IP Office is set correctly, then all unanswered calls will go to the IP Office Voice Mail Box. It makes no difference how the Timeouts are set when the cell phone is turned off.