Tone dialling, also called Dual
Tone Multi-Frequency (DTMF) or TouchTone, is a
relatively recent development. It is now accepted by virtually
all exchanges and will eventually supersede pulse dialling
Each digit is represented by a combination of two tones from
seven. Figure 3 shows a telephone key pad and the tones used.
Included in Figure 3 are the special keys A, B, C and D. These
are used in some systems to perform special functions. They
are included for completeness and will not be discussed here.
When a key is pressed the two tones which cross
at the key are transmitted to the exchange. E.g. dialling
a 1 will send 697Hz and 1209Hz together, 2 will send 697Hz
and 1336Hz and so on. Each digit is represented by a unique
combination of tones. DTMF digits can be sent and detected
more quickly than pulse digits. A detection time of only 50ms
with an interdigit time of 50ms means that any digit can be
dialled in 100ms. Compared to a pulse dialled 1 taking 800ms
and pulse 0 taking 1.7sec.
DTMF dialling is therefore much faster than pulse dialling.
The tones as they are transmitted give a distinctive chirping
sound in the earpiece as they are sent.
To indicate the presence of an incoming call the exchange
must make the telephones connected ring. This is done using
a ring generator at the exchange that sends a 75Vac signal
between 20Hz and 47Hz to the subscriber end. This triggers
the ringer in the telephone.
Ringing is sent to the called party first, then the calling
party. This allows the exchange ring generator to make most
efficient use of the power required to ring the handsets.
It also means the caller has ringing around 2 seconds before
the calling party hears it. This can lead to the called party
answering before the calling party even hears a ring tone
if they are quick.
Another important function of the exchange is to inform the
caller of the progress, success and failure of their call.
this is performed using a series of tones. Each is different
and indicates a specific condition. These are:
||OK to proceed with dialling
||The called party equipment is in use
||The called party equipment is ringing
||The dialled number does not exist or cannot
The precise nature of these tones varies between networks
and systems. Modern equipment has replaced some with spoken
messages giving a more friendly and understandable message.