Yealink SIP-T54S Administrator's Manual page 1100

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Step
F9 INVITE—User C to Proxy Server
F10 INVITE—Proxy Server to User A
180 Ringing—User A to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User C
F13 INVITE—User A to Proxy Server
F14 INVITE—Proxy Server to User B
F15 200 OK—User B to Proxy Server
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Action
User C sends a SIP INVITE message to the
proxy server. The INVITE request is an
invitation to User A to participate in a call
session.
In the INVITE request:






The proxy server maps the SIP URI in the
To field to User A. The proxy server sends
the INVITE message to User A.
User A sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
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.
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.
The proxy server forwards the mid-call
INVITE message to User B.
User B sends a 200 OK to the proxy server.
The 200 OK response indicates that the
Description
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.