Dial Tone: This tone is used to indicate that the exchange is ready to accept dialed digits from the subscriber. The subscriber should start dialing only after hearing the dialing tone. Sometimes, however, a few seconds may elapse before the dial tone is heard. This happens particularly in common control exchanges which use shared resources for user interfaces. The dial tone is a 33 Hz or 50 Hz or 400 Hz continuous tone as shown in Fig. The 400 Hz signal is usually modulated with 25 Hz or 50 Hz.
When the called party line is obtained, the exchange control equipment sends out the ringing current to the telephone set of the called party. This ringing current has the familiar double-ring pattern. Simultaneously, the control equipment sends out a ringing tone to the calling subscriber, which has a pattern similar, that of the ringing current as shown in Fig. The two rings in the 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. The frequency of the ringing tone is 133 Hz or 400 Hz,
Busy tone pattern is shown in Fig. It is a bursty 400 Hz signal with silence period in between. The burst and silence duration have the same value of 0.75 s or 0.375 s. A busy tone is sent to the calling subscriber whenever the switching equipment or junction line is not available to put through the call or the called subscriber line is engaged. Fig. shows the number unobtainable tone which is continuous 400 Hz signal. This tone may be sent to the calling subscriber due to a variety of reasons such as the called party line is out of order or disconnected, and an error in dialing leading to the selection of a spare line. In some exchanges the number unobtainable tone is 400 Hz intermittent with 2.5 s on period and 0.5 s off period.
The routing tone or call-in-progress tone is a 400 Hz or 800 Hz intermittent pattern. In electromechanical systems, it is usually 800 Hz with 50 percent duty ratio and 0.5 s on/off period. In analog electronic exchanges it is a 400 Hz pattern with 0.5 s on period and 2.5 s off period. In digital exchanges, it has 0.1 s on/off periods at 400 Hz. When a subscriber call is routed through a number of different type of exchanges, one hears different call-in-progress tones as the call progresses through different exchanges. Fig. shows a routing tone pattern.