Types of Backward Signal

Dial Tone Signal






-The Dial Tone signal is used to indicate that the switching  equipment has recognized that a telephone has gone off-hook, and   the switching equipment is prepared to receive the dialed digits or  DTMF codes.
-The tone stops when the first number is dialed
 -Dial tone is a continuous tone having frequency of 425 Hz in Malaysia                                        


Ringing Signal

-The telephone exchange will issue a ringing signal to acknowledge subscriber for incoming call.
-It is a double ring, repeated regularly. It has a low frequency of   20Hz or 25Hz with a cadence of 0.4sec on 0.2sec off 0.4sec on  2.0sec off.




Ringing Tone Or Ring Back Tone

-The telephone exchange will issue a ringing tone to acknowledge the caller that the call is connected   is a double ring, repeated  regularly.
- It has a frequency of 425Hz with a cadence(modulation)of                                                                      0.4second on 0.2second off 0.4second on 2.0second off


Busy Tone Signal

-The telephone exchange will issue a busy tone to the calling subscriber that indicates failure to complete the requested connection.
 - Meaning that, at that moment the called subscriber is in off-hook                                                           state. Busy tone is also known as engaged tone.
 -Busy tone has a frequency of 425Hz and equal length on and off                                                             tones (0.5second on 0.5second off).


Number Unobtainable Tone 
   -It is a voice message issued by telephone exchange when                                                                        the dialed  number is no longer in the service.
                                                   

                                    example : ‘The number you have dialed is not in service’