S 4.37 Blocking fax sender numbers

Initiation responsibility: IT Security Officer

Implementation responsibility: Person in Charge of the Fax System, Fax Mail Centre

To prevent certain fax transmissions from blocking the organisation's own fax machine, e.g. from being overloaded with by special fax advertising campaigns, it is possible to block certain fax sender numbers if necessary.

Several modern fax machines (Group 4) are able to evaluate the sender numbers transmitted and thus refuse fax transmissions from particular numbers. This also applies to some fax servers, provided that they are connected to the ISDN network. In addition, the Call Subscriber ID (CSID) can also be used to evaluate the numbers from which incoming faxes are sent. Unfortunately, however, it is possible that the fax sender can suppress the transmission of their fax numbers and manipulate the number transmitted and the subscriber ID.

Another alternative is to configure a closed user group with the telephone network operator, subject to a charge, if sender and recipient are connected to digital exchanges. This alternative is also offered by modern PBX systems (see also module S 3.401 Telecommunications system).

Review questions:

• If certain fax sender numbers are to be blocked: Are certain fax sender numbers blocked on the fax machine, fax server, on the PBX system or by the network operator and/or only certain fax numbers allowed?