Yealink SIP-T4X Administration Manual page 426

Step Action
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
User B
200 OK—User B to Proxy
Server
F7
200 OK—Proxy Server to User
A
F8
Description
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
User A wants to disconnect the call.
User B sends a SIP 200 OK response to
the proxy server. The SIP 200 OK
response indicates that User B has
received the CANCEL request.
The proxy server forwards the SIP 200
OK response to notify User A that the
CANCEL request has been processed
successfully.
Appendix
411