This is a read-only counter, in milliseconds, recorded in the call history table for the duration relevant to computing the Voice Utilization Rate of a completed call leg, obtained by dividing this value by cvCallHistoryTxDuration. An admin uses this historical figure to retroactively assess how much of a finished call's transmit-path time was actually carrying voice, mirroring the active-call equivalent but for calls that have already ended. For example, investigating a past call flagged for poor quality, an admin would compute this ratio to see whether the call was mostly silence or genuinely full of voice traffic.