Let's consider that you have an autonomous (running even when your PC is
off) dial-up fax modem and a classical answering machine on the same
analogue line. Here's the problem : if you want your classical answering
machine take care of vocal messages and your PC store incoming fax
messages, there is no way to distinguish nature of the incoming call
before hanging up, ringings are the same. And the matter is, two devices
which count both the number of ringings are incompatible on the same
line, because the first of the two devices which will hang up will keep
the call for itself, whatever the nature of this call.
And the solution is the device you're talking about. You connect it in
first position to the phone line, and it directs calls to the
appropriate device. You can find one at
http://www.comm-inter-tel.com/fr/accessoire/divers.html (french only,
but I guess they would answer an e-mail in english). It's the last
device listed on the page.
Another idea is to have a look on an auction site like ebay or yahoo.
One or two months ago, exactly what you are looking for was for sell on
ebay.fr (as you may guess, english isn't my native tongue :)
Several devices exist and many are built-in fax machines. For example,
my Panasonic KX-FP85 can answer the phone, and either record a voice
message or automatically receive a fax. I can also answer from any
other phone in the house and direct the call to the fax machine via
two DTMF bursts.
Stand-alone devices really answer the call, so be careful. They
essentially answer and send out phony ringing tones to the caller. If
the device hears the CNG tone of a fax machine, they "ring" the
attached fax. On the other hand, if after a few seconds, it does not
hear the CNG tone, then it rings the attached telephone.
Making one is certainly possible, but why go through the effort? After
all, you will also need to factor in PSTN requirements to protect
yourself, their employees and equipment. I doubt that you are
concerned about "certification", but if you mess-up, you could be
disconnected and fined.