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The modem itself cannot distinguish between an incoming fax/data/voice call.  
With software, it is possible to discriminate between a fax/data/voice call.

Kolja Waschk (kawk@Yo.COM) [writer of the ZuTSR and ZUtil] writes:
"The modem itself cannot discriminate between a fax/data/voice call.  Voice 
calls do not differ from normal data calls (assuming most calling modems do 
not send a calling tone).

However, the modem can be told to answer in Voice Mode, and then "listen"
what's on the line. It can detect several possibilities, ie. "fax calling 
tone heard", "modem calling tone heard" (since firmware 6.11), "DTMF tone 
Nr.x heard", and, while in record mode, "caller keeps silent" or "caller 
made some noise and then turned silent". It's the software on the computer 
that should react differently on these reports and switch the modem into 
data/fax mode respectively. Two programs are known to do more : VoiceConnect 
and the (not yet released) new ZUtil do real-time signal analysis on the 
incoming voice data and can detect - in addition to the above mentioned 
events - the presence of "human voice".

The most used method to distinguish between data and voice calls is the
following (flow controlled by the software on the computer)

1. answer the line
2. replay play a spoken greeting message, provoking voice callers to speak
   (a small "Hello?" does the job)
3. listen
4. switch to data/fax mode if a calling tone has been received
5. switch to data/fax mode if the caller keeps saying nothing [The call is not
   conclusively proven to be a data call, but it is reasonably assume to be a
   data call.  Ed.]
6a. handle the voice call if "voice" (only with VC or ZUtil) has been
    detected or simply the caller does not remain silent and does not
    sent calling tones.
6b. handle the voice call if the caller requests it by sending a DTMF
    tone, otherwise switch to data/fax mode

As you might see, the spoken greeting message is quite required to provoke a
reaction of voice callers. Because there is no space yet in the modem's RAM
preserved for storing a greeting message, the above method cannot be
automated and done by the modem stand-alone.

This makes it impossible for communications software - which has not been
designed to do so - to distinguish between data and voice calls. (Anyway, for
software like a FidoNet mailer that uses the FOSSIL serial driver, a tricky
TSR utility exists to do the job - ask me, the author ;-)"

N.B.: This is a new entry.  As far as the Keeper of the ZyXEL FAQ knows, it
      is factually correct.