Configuring Audio Features
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signaling over analog telephone lines in the voice-frequency band. DTMF is the signal sent
from the IP phone to the network, which is generated when pressing the IP phone’s keypad
during a call. Each key pressed on the IP phone generates one sinusoidal tone of two
frequencies. One is generated from a high frequency group and the other from a low frequency
group.
The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and
each column representing a high frequency. Pressing a digit key (such as '1') will generate a
sinusoidal tone for each of two frequencies (697 and 1209 hertz (Hz)).
DTMF Keypad Frequencies:
Note
Methods of Transmitting DTMF Digit
Three methods of transmitting DTMF digits on SIP calls:
RFC 2833 -- DTMF digits are transmitted by RTP Events compliant to RFC 2833.
INBAND -- DTMF digits are transmitted in the voice band.
SIP INFO -- DTMF digits are transmitted by SIP INFO messages.
The method of transmitting DTMF digits is configurable on a per-line basis.
RFC 2833
DTMF digits are transmitted using the RTP Event packets that are sent along with the voice
path. These packets use RFC 2833 format and must have a payload type that matches what the
other end is listening for. The default payload type for RTP Event packets is 101 and the
payload type is configurable. The IP phones use the configured value to negotiate with the
other end during call establishment.
The RTP Event packet contains 4 bytes. The 4 bytes are distributed over several fields denoted
as Event, End bit, R-bit, Volume and Duration. If the End bit is set to 1, the packet contains the
end of the DTMF event. You can configure the sending times of the end RTP Event packet.
INBAND
DTMF digits are transmitted within the audio of the IP phone conversation. It uses the same
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