Analog Address Signaling
The dialing phase allows the subscriber to enter a phone number (address) of a telephone
at another location. The customer enters this number with either a rotary phone that generates
pulses or a touch-tone (push-button) phone that generates tones. Table 3-2 shows
the frequency tones generated by dual tone multifrequency (DTMF) dialing.
Table 3-2 DTMF Frequencies
Frequencies 1209 1336 1477
697 1 2 3
770 4 5 6
852 7 8 9
941 * 0 #
Telephones use two different types of address signaling to notify the telephone company
where a subscriber calls:
■ Pulse dialing
■ DTMF dialing
These pulses or tones are transmitted to the CO switch across a two-wire twisted-pair
cable (tip and ring lines). On the voice gateway, the FXO port sends address signaling to
the FXS port. This address indicates the final destination of a call.
Pulsed tones were used by the old rotary phones. These phones had a disk that was rotated
to dial a number. As the disk rotated, it opened and closed the circuit a specified number
of times based on how far the disk was turned. The exchange equipment counted
those circuit interruptions to determine the called number. The duration of open-toclosed
times had to be within specifications according to the country you were in.
These days, analog circuits use DTMF tones to indicate the destination address. DTMF
assigns a specific frequency (consisting of two separate tones) to each key on the touchtone
telephone dial pad. The combination of these two tones notifies the receiving subscriber
of the digits dialed.
Informational Signaling
The FXS port provides informational signaling using call progress (CP) tones, as detailed
in Table 3-3. These CP tones are audible and are used by the FXS connected device to
indicate the status of calls.
Table 3-3 Network Call Progress Tones
Tone Frequency (Hz) On Time (sec) Off Time (sec)
Dial 350 + 440 Continuous Continuous
Busy 480 + 620 0.5 0.5
Ringback, line 440 + 480 2 4
Ringback, PBX 440 + 480 1 3
Congestion (toll) 480 + 620 0.2 0.3
Reorder (local) 480 + 620 0.3 0.2
Receiver off-hook 1400 + 2060 + 2450 + 2600 0.1 0.1
No such number 200 to 400 Continuous Continuous
The progress tones listed in Table 3-3 are for North American phone systems.
International phone systems can have a totally different set of progress tones. Users
should be familiar with most of the following call progress tones:
■ Dial tone: Indicates that the telephone company is ready to receive digits from the
user telephone.
■ Busy tone: Indicates that a call cannot be completed because the telephone at the
remote end is already in use.
■ Ring-Back (normal or PBX): Tone indicates that the telephone company is attempting
to complete a call on behalf of a subscriber.
■ Congestion: Progress tone is used between switches to indicate that congestion in
the long-distance telephone network currently prevents a telephone call from being
processed.
■ Reorder: Tone indicates that all the local telephone circuits are busy and thus prevents
a telephone call from being processed.
■ Receiver off-hook: Tone is the loud ringing that indicates the receiver of a phone is
left off-hook for an extended period of time.
■ No such number: Tone indicates that the number dialed cannot be found in the routing
table of a switch.