When the fax number is showing “unknown,” it is being caused by the sender. eFax captures the TSID / CallerID info when presented to them from the telco (other fax provider). If that info is hidden, there is nothing to report for the fax number which is why it comes up as “unknown.” You can base your list view logic or process on fax number provided in the “Caller ID” field instead of this fax number field as a workaround.