Yealink SIP-T2 SERIES Administrator's Manual page 1012

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
Step
INVITE—User C to Proxy
F9
Server
INVITE—Proxy Server to User
F10
A
180 Ringing—User A to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User C
INVITE—User A to Proxy
F13
Server
INVITE—Proxy Server to User
F14
B
F15
200 OK—User B to Proxy
990
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
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.