Informational Signaling
DTMF tones are used not just for
address signaling but also for informational signaling. Specifically,
call-progress indicators in the form of tone combinations are used to notify
subscribers of call status. Each combination of tones represents a different
event in the call process, as follows:
-
Dial tone Indicates that the
telephone company is ready to receive digits from the user telephone. Cisco
routers provide dial tone as a method of showing that the hardware is installed.
In a PBX or key telephone system, the dial tone indicates the system is ready to
receive digits.
-
Busy Indicates that a call
cannot be completed because the telephone at the remote end is already in
use.
-
Ringback (CO or PBX) Indicates
that the telephone switch is attempting to complete a call on behalf of a
subscriber.
-
Congestion Indicates that
congestion in the long-distance telephone network is preventing a telephone call
from being processed. The congestion tone is sometimes known as the all-circuits-busy tone.
-
Reorder Indicates that all of
the local telephone circuits are busy, thus preventing a telephone call from
being processed. The reorder tone is known to the user as fast-busy and is familiar to anyone who operates a
telephone from a PBX.
-
Receiver off hook Indicates
that the receiver has been off hook for an extended period without placing a
call.
-
No such number Indicates that
a subscriber placed a call to a nonexistent number.
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