Note that US Robotics calls this "Call Selection" rather than Adaptive Answering. Also, the USR Sportster 14.4K modem does NOT have Call Selection.
There are two methods a modem can use to determine whether the incoming call is a fax machine or not.
The first method is very reliable when receiving from fax modems and junk fax machines, but sometimes does not work very well when the remote is a real fax machine. Why? Because many real fax machines do not send out a CNG tone until the user hits the "START" button. Many fax machine users will not hit the "START" button until they hear "noises" from the other end. In the case of a fax modem using the first method of adaptive answering, the modem will have already listened for the CNG tone and is now committed to data mode only (i.e., the fax call is missed). If this turns out to be a problem, you can call the remote fax operator voice, and tell them to hit their "START" button IMMEDIATELY after dialing the last digit of your BBS telephone number.
The second both is much more reliable, as long as the modem manufactuer implemented it correctly in the modem firmware. Many Rockwell V.FC and V.34 modems use the second method, but the modems mistake many true data calls as fax calls due to buggy code. Supra has released a FLASH ROM upgrade for their V.FC modems to fix the Rockwell bug. Zoom and many other Rockwell-based modem manufactuers, as far as I know, are still selling buggy V.FC and V.34 modems (with respect to adaptive answering).