This is a read-only counter, in milliseconds, recorded in the call history table for how long fax data was transmitted during a now-completed call leg, used to compute the FAX Utilization Rate by dividing this value by cvCallHistoryTxDuration. An admin uses this historical figure to confirm after the fact whether a call believed to be a fax transmission actually carried fax data for a meaningful portion of its duration. For example, an admin reviewing failed fax delivery reports could check this history object to see whether the call even negotiated meaningful fax transmission time before it ended.