This is a read-only counter, in milliseconds, tracking how long fax data has been transmitted from this peer to the voice gateway during the call leg, again locking at roughly two days rather than wrapping. An admin divides this by cvCallActiveTxDuration to obtain the FAX Utilization Rate for the call leg, which helps confirm whether a call believed to be carrying fax traffic is actually doing so. For example, an admin investigating a call classified as fax but showing near-zero value here would suspect the fax negotiation failed and voice-band data is being sent instead.