This is the number of milliseconds of fax data transmitted from this peer to the gateway during the call. Dividing this by hh3cVoCallActiveTxDuration yields the fax utilization rate, letting an admin confirm whether a call carried fax traffic, such as a T.38 fallback, rather than voice. For instance, a fax tx duration of 40000 ms out of a 45000 ms total call indicates the call was almost entirely a fax transmission.