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Administrator’s Guide for SIP-T3xG IP Phones
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There are 3 common 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 the SIP INFO messages.
The method of transmitting DTMF digits is configurable on a per-account 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 payload type for the RTP Event packets
is configurable. The IP phone often defaults to 101 for the payload type, which uses your
definition 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 number of times for
the IP phone to send the RTP Event packet with End bit set to 1. The number of times is 3
by default.
INBAND
DTMF digits are transmitted within the audio of the IP phone conversation. It uses the
same VoIP codec as your voice and is audible to the conversation partners.
SIP INFO
DTMF digits are transmitted by the SIP INFO messages when the voice stream is
established after a successful SIP 200 OK-ACK message sequence. The SIP INFO
message is sent along the signaling path of the call. The SIP INFO message can support
transmitting DTMF digits in three ways: DTMF, DTMF-Relay and Telephone-Event.
Procedure
Configuration changes can be configured using the configuration files or locally.
Configuration File
<MAC>.cfg
Configure the method of
transmitting DTMF digit and the
payload type.
For more information, refer to
DTMF on page 270 .
<y0000000000xx>.cfg
Configure the number of times
for the IP phone to send the end
RTP Event packet.
For more information, refer to