Administrator’s Guide for SIP-T2xP IP Phones
48
Callee ID Presentation
on page
133
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DTMF (Dual Tone Multi-frequency), better known as touch-tone, is used for
telecommunication signaling over analog telephone lines in the voice-frequency band.
DTMF is the signal sent from the IP phone to the network that you generate when
pressing the phone’s keypad during the call. Each key you press on the 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:
1209 Hz
1336 Hz
1447 Hz
1633 Hz
697 Hz
1
2
3
A
770 Hz
4
5
6
B
852 Hz
7
8
9
C
941 Hz
*
0
#
D
There are 3 common methods of transmitting DTMF 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 (G.711).
SIP INFO -- DTMF digits are transmitted by the SIP INFO messages.
The method of transmitting DTMF is configurable on a per-account basis.
RFC 2833
DTMF digits are transmitted using special IP 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 IP phone often defaults to 101 for the payload
type, which uses your definition to negotiate with the other end during call
establishment.
INBAND
DTMF digits are transmitted within the audio of the phone conversation. It uses the
same VoIP codec as your voice and is audible to the conversation partners.
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