  SINGLE TONE

  This option Enables/Disables Single tone. This option, when enabled,
  allows the user to transmit a tone of selected frequency and duration.
  The singletone mode is entered by pressing and holding the RAT button
  (lower of two small buttons on the side of the radio).  The keypad is now
  in the singletone mode.  Pressing one of the keys on the keypad will cause 
  the radio to go into transmit mode, emit a 1640 Hz side tone (if enabled)
  and transmit a singletone corresponding to the key pressed. 
  NOTE: Singletone can only be enabled if SIGNALLING TYPE is set to NONE
  (Digital Signalling feature).  
