The controller then initiates a re-INVITE(Message 11) to instruct the caller’s
UA in the re-INVITEmessage where to direct the DTMF media stream using
the SDP connection data to the DTMF collector acquired in Message 3. This re-
INVITEhas the following form:INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 100.101.102.103;branch=z9hG4bK7d
To: User B <sip:[email protected]>
From: User A <sip:[email protected]>;tag=19023023
Call-ID: [email protected]
Max-Forwards: 70
CSeq: 1 INVITE
Contact: s<ip:[email protected]>
Content-Type: application/sdp
Content-Length: ...v=0
o=UserB 289375749 289375749 IN IP5 100.101.102.103
s=-
c=IN IP4 100.101.102.103
t=0 0
m=audio 5004 RTP/AVP 0
m=audio 53000 RTP/AVP 96
c=IN IP4 200.201.202.203
a=rtpmap:96 telephone-eventNote that this SDP now has a second media m=line for the DTMF digit
transport with a new connection c=line with the IP address of the DTMF digit
collector. The caller can now send DTMF digits in mid-call to the digit collec-
tor, since it knows the connection data to the DTMF controller.
The called party may instruct the caller to input data using the telephone
keypad. The resulting DTMF digits are captured by the DTMF collector and
sent to the controller in the HTTPGETmessage (15).
Plain DTMF service is useful for simple applications such as two-stage dial-
ing, where the user first dials an access number for the respective service, gets
a prompt tone, and then dials an identification such as the calling card num-
ber. A new dial tone invites the user then to dial the phone number. As we will
see, DTMF digits can also be collected by more complex interactive voice
response systems.332 Chapter 19