This article explains why some inbound faxes may display 'UnknownCaller' as the source when reviewed from the FaxMaker Online usage reports as shown below:
Inbound faxes having UnknownCaller as the source occurs due to the sender blocking their Caller ID/CSID.
Faxmaker Online has no control over the Caller ID/ANI that is passed to us – or is prevented from being passed to us – by the sender fax appliance or service.
It is a fairly common practice for some senders to block caller ID, or out pulse a number that differs from the return fax number. When this happens, FaxMaker Online is unable to determine the sending CSID consequently showing the source as UnknownCaller.
ANI: Automatic Number Identification is used by the phone carriers to determine the originating fax or telephone number for any call. A caller’s telephone number is captured by the carrier's ANI service even if caller ID blocking is activated.