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