I was just searching this information, and found neat way to setting up a Linux as an answering, and fax machine.
Because many small businesses and startups don’t have the resources that the big companies have, like a dedicated receptionist or a 24/7 call centre. But with the help of some technology and good taste you can get a long way to help your (potential) customers when you are out of the office.
VOCP system is much more than an answering machine, VOCP transforms your computer into a full-featured call answering and voice messaging system. With the VOCP System, you can create an unlimited number of voicemail, pager and command shell boxes which callers will navigate using their touch tone telephone. You can send and receive faxes, listen to your email using text-to-speech, filter and redirect calls based on caller ID information, run programs through the telephone and more. It should work on Linux, BSD, Solairs systems.