The call duration on (mobile) phones starts when the call is connected to the IVR whereas the call lands on IVR few seconds prior to getting connected and the caller mostly listens to a ringing tone.

The call duration on IVR stops when all the data processing is done on the IVR and this generally takes a couple of seconds post the disconnection of the call by the caller.

Both the events create a difference in the duration the callers see on their mobiles and on the Logs panel. This difference is also permissible in the system to compensate the duration difference with telecom provider and is less than 10 seconds in duration.