Phone Features
33
DTMF
The phone provides 2 mechanisms for transmitting ! DTMF information,
inband and DTMF in RTP (see RFC 2833). The phone does not support out-
band DTMF through SIP messaging. There are no configuration parame-
ters on the phone which control the use of DTMF.
A process of negotiation is used during call-setup to determine which form
of DTMF signaling will be used. The phone supports send DTMF informa-
tion in response to the user pressing the keys 0-9 and * and # when in a
call connected state.
When a call is made from a phone it will "Offer" the remote endpoint sup-
port for DTMF in RTP (this is carried in the SDP protocol). If the far end
does not "answer" that it can support DTMF in RTP then DTMF in-band will
be used otherwise DTMF in RTP will be used.
When DTMF in RTP is negotiated the phone will always "offer" payload 100
to carry the DTMF events. The far end may accept and confirm this payload
or it may suggest a different payload value. In this case the phone will fol-
low that payload preferred by the far end. On an incoming call the phone
will follow the payload value suggested by the far end.
The phone is not capable of retrieving or understanding DTMF in-band or
DTMF in RTP information it may receive. This information is normally used
by application or media servers to control feature access. If the user press-
es keys when in a call connected state and in-band DTMF has been nego-
tiated he will hear the tones being sent in the speech path (handset only).
If DTMF in RTP has been negotiated he will here clicks as speech packets
are removed and replaced with DTMF in RTP key events.
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