This is a read-only counter, in milliseconds, tracking how long the transmit path has been open from this peer to the voice gateway for the current call leg, and it locks at its maximum value after roughly two days rather than wrapping. An admin monitors this to see how long a call leg has actually had its transmit path active, which is the denominator used to compute voice or fax utilization rates from the related duration counters. For example, dividing the voice-transmit-duration counter by this value would give the admin the Voice Utilization Rate for that call leg.