If however, a call is generated by a fax machine on an ISDN line, the SETUP message will describe the call type as being specifically from a "G3 Fax".

In this case, the destination fax machine will not answer the call as it is configured to accept only analogue calls.

In practice, a device can only apply one filter to each parameter of the incoming SETUP. A filter such as "CallType=G3Fax OR CallType=Analogue" is not normally possible.

For these reasons, this solution cannot be recommended. It would work very reliably if all networks were ISDN; and all Fax machines placed G3 Fax type calls and they were configured to answer such calls.

Fortunately, there is alternative method for directing incoming calls to the correct device - Multiple Subscriber Numbering, or MSN.