Yealink T46G Administrator's Manual page 405

Step Action
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
Description
session is now active.
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 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.
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
Appendix
391