Yealink SIP-T19 E2 T4 Series, Administrator'S Manual

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Appendix
933
Step Action Description
session.
In the INVITE request:

The IP address of User A is inserted in
the Request-URI field.

User C is identified as the call session
initiator in the From field.

A unique numeric identifier is
assigned to the call and is inserted in
the Call-ID field.

The transaction number within a
single call leg is identified in the CSeq
field.

The media capability User C is ready
to receive is specified.

The port on which User A is prepared
to receive the RTP data is specified.
F10 INVITE—Proxy Server to User A
The proxy server maps the SIP URI in the To
field to User A. The proxy server sends the
INVITE message to User A.
F11
180 Ringing—User A to Proxy
Server
User A sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the user is being alerted.
F12
180 Ringing—Proxy Server to
User C
The proxy server forwards the 180 Ringing
response to User C. User C hears the
ring-back tone indicating that User A is
being alerted.
F13 INVITE—User A to Proxy Server
User A sends a mid-call INVITE request to
the proxy server with new SDP session
parameters, which are used to place the call
on hold.
F14 INVITE—Proxy Server to User B
The proxy server forwards the mid-call
INVITE message to User B.
F15 200 OK—User B to Proxy Server
User B sends a 200 OK to the proxy server.
The 200 OK response indicates that the
INVITE was successfully processed.
F16 200 OK—Proxy Server to User A
The proxy server forwards the 200 OK
response to User A. The 200 OK response
notifies User B is successfully placed on