A call takes place when both parties are off the hook. In circuitry, a closed-loop is established by the time the recipient picks up the phone. Only when there is a closed-loop that the information is transmitted through the medium. By the time any of the two parties disconnects the call, by putting the phone back on hook, there is no more data transmitted. Hence, it concludes the call. The call duration is simply the duration of the call being established between the caller and recipient, in which an actual conversation took place.