The call duration feature in the Live call widget was implemented to help you know the average duration of your ongoing calls. The duration shown is just an approximation that is generated by the server. So, if you see a slight difference after you disconnect the call, it is because the duration shown is not the actual one, but an estimation to help you know for how much time the call lasted.

How do we calculate billing in this case?

The billing of our customers depends on the call duration that is shown in the logs section of the panel and not on the basis of the call duration that is shown in Live Call Transfer widget.