The Free On-line Dictionary of Computing (30 December 2018):

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, though V.24 only
   specifies the meaning of the signals, not the connector or the
   voltages used.

   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

   (2004-08-02)