Q. What are the various tones used in stronger telephony?

When called party line is obtained, exchange control equipment transmits out the ringing current to telephone set of the called party. This ringing current has familiar double-ring pattern. Concurrently, control equipment sends out a ringing  tone to  calling  subscriber,  which  has  a  pattern  similar,  that  of  the ringing  current  as  demonstrated in  Figure.  The two  rings  in  double  ring  pattern  are separated by a time gap of 0.2 s and two double ring patterns by a gap of 2 s. The ring burst has duration of 0.4 s. Frequency of the ringing tone is 133 Hz or 400 Hz, Busy tone pattern is illustrated in Figure below. It is a burst 400 Hz signal with silence period in between. Burst and silence duration have the same value of 0.75 s or 0.375 s. A busy  tone  is  sent  to  calling  subscriber  whenever  switching  equipment  or junction line isn't available to put through the call or called subscriber line is engaged. Figuredemonstrates the number unobtainable tone that is continuous 400 Hz signal. This tone may be sent to calling subscriber because of a variety of reasons like the called party line is out of order or disconnected, and an error in dialling resulting in the selection of a spare line. In some exchanges number unobtainable tone is 400 Hz intermittent with 2.5s on period and 0.5 s off period. Routing  tone  or  call-in-progress  tone is  a  400  Hz  or  800  Hz  intermittent pattern.  In  electromechanical  systems,  it  is  generally 800  Hz  with  50  percent  duty ratio and 0.5 s on/off period. In analog electronic exchanges it's a 400 Hz pattern with 0.5s on period and 2.5 s off period. Whereas in digital exchanges it has 0.1 s on/off periods at 400 Hz. When a subscriber call is routed via a number of different type of exchanges, one hears different call-in-progress tones as call progresses by different exchanges. Figureillustrates a routing tone pattern.