Yealink SIP-T48G Administrator's Manual page 585

Step Action
INVITE—Proxy Server to User
F2
B
100 Trying—User B to Proxy
F3
Server
100 Trying—Proxy Server to
F4
User A
180 Ringing—User B to Proxy
F5
Server
180 Ringing—Proxy Server to
F6
User A
200 OK— User B to Proxy
F7
Server
200OK—Proxy Server to User
F8
A
Description
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. The proxy server
sends the INVITE message to User B.
User B sends a SIP 100 Trying response
to the proxy server. The 100 Trying
response indicates that the INVITE
request has been received by User B.
The proxy server forwards the SIP 100
Trying to User A to indicate that the
INVITE request has been received by
User B.
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the User B 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 B sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies User A that the connection has
been made.
The proxy server forwards the 200 OK
message to User A. The 200 OK
Appendix
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