This is a read-only counter, in milliseconds, tracking how long actual voice audio has been transmitted from this peer to the voice gateway during the call leg, and like its sibling counters it locks at roughly two days rather than wrapping. An admin divides this by cvCallActiveTxDuration to compute the Voice Utilization Rate, revealing what fraction of the open transmit path is carrying real voice versus silence or other non-voice content. For example, a low ratio between this counter and the transmit duration could indicate the call is mostly silence, possibly pointing to a stuck or idle line.