Source : Free On-Line Dictionary of Computing

V.24
     
         The {ITU-T} {standard} defining interchange
        circuits between {DTE} and {DCE}.  V.24 is the {ITU-T}
        equivalent of {EIA} standard {EIA-232C}.
     
        V.24 recommends 12 {modem} {carrier} frequencies that will not
        interfere with {Dual Tone Multi-Frequency} or other telephone
        control tones.  These are:
     
          GROUP A   =    920 Hz, 1000 Hz, 1080 Hz, 1160 Hz
          GROUP B   =    1320 Hz, 1400 Hz, 1480 Hz, 1560 Hz
          Group C   =    1720 Hz, 1800 Hz, 1880 Hz, 1960 Hz
     
        (1995-03-02)