The fax machine on the analogue network makes a call to the fax machine on
the ISDN network.
The SETUP generated on receipt of the call by the ISDN network specifies the
call type is Generic Analogue. There must be a call type in a SETUP message, but
the ISDN cannot know exactly what kind of device is causing this SETUP,
so it makes a best guess.
In this case, the fax machine in the bottom-right corner will happily answer
the incoming call.