This is a read-only counter, in milliseconds, recorded in the call history table showing how long the transmit path was open from this peer to the voice gateway for a now-completed call leg, and it locks at roughly two days maximum rather than wrapping. An admin uses this historical figure the same way as the active-call version, as the denominator for computing voice and fax utilization rates, but for a call that has already ended. For example, an admin reviewing a completed call's quality after a complaint would divide the corresponding voice or fax duration by this value to compute utilization rates retroactively.