This read-only TimeStamp records the start time of the call, specifically the moment the Setup Message (or Add Party Message) was first received on this interface. An administrator uses this to determine how long a given call has been active, which is useful both for capacity/usage reporting and for correlating a call's start against other events like a configuration change or an outage. For example, an operator investigating a burst of failures compares cwspCallStartTime across active calls to see whether the affected calls all started shortly after a known network change.