Yealink SIP T46G Administrator's Manual page 392

Step Action
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
180 Ringing—User B to Proxy
Server
F3
180 Ringing—Proxy Server to
User A
F4
CANCEL—User A to Proxy
Server
F5
CANCEL—Proxy Server to
F6
Description
User A sends an INVITE message to a
proxy server. The INVITE request is an
invitation to User B to participate in a
call session.
In the INVITE request:
The IP address of User B is inserted

in the Request-URI field.
User A 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 A is

ready to receive is specified.
The port on which User B is

prepared to receive the RTP data is
specified.
The proxy server maps the SIP URI in the
To field to User B. Proxy server forwards
the INVITE message to User B.
User B 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 A. User A
hears the ring-back tone indicating that
User B is being alerted.
User A sends a SIP CANCEL request to
the proxy server after not receiving an
appropriate response within the time
allocated in the INVITE request. The SIP
CANCEL request indicates that User A
wants to disconnect the call.
The proxy server forwards the SIP
CANCEL request to notify User B that
Appendix
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