I've wired up my house to have a phone socket in each room with the cabling
going to a central point under the stairs, where I have a home PABX
installed. I'd like now to connect a fax machine and an answering machine
(or TAD as these machines seem to be called). The PABX can be programmed so
that it knows on which extensions the fax and TAD are attached to. However,
in this case the answering machine connection cannot be used as a phone, and
since I have a DECT phone/answering machine combo, this means that I can't
internally "phone" this extension.

The fax instructions state that I should connect the fax and any downstream
devices in series, with the fax closest to the cable coming into the house
from the exchange. I looked inside the fax machine, and noticed that all 4
wires on the cable seem to be used, so I assume that the fax machine is
smart enough to switch the call downstream in some way if it decides that
the incoming call is a voice call.

Does anyone know about this, or can point me somewhere?

Many thanks in advance,
Paul