What you're asking for is inherently impossible, since the fax tone can't be detected until AFTER the phone is answered. However, there are devices that can simulate the behavior you've described. You can get a box that plugs into your phone line and into your phone devices (fax modem, phones, answering machine, etc.). The box answers each incoming call on the first ring, and it listens for fax tone, data tone, etc. Then it re-routes the call to the appropriate device. The box generates an artificial ring when re-routing so the device knows it's being signalled. Some of them also generate a feedback ring to the caller on a voice call while ringing your phone so the caller doesn't think something has gone wrong.
Last time I looked these went for around $75 to $150 depending on complexity. You might try your local Circuit City, Fry's, or equivalent.
I hope this helps.
Steve Diamond