Yealink SIP-T48G Administrator's Manual page 314

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
Step
INVITE—Proxy Server to User
F2
B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
200 OK—User B to Proxy
F5
Server
200 OK—Proxy Server to User
F6
A
F7 ACK—User A to Proxy Server
F8 ACK—Proxy Server to User B
302
Action
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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 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
response notifies User A that the
connection has been made.
User A sends a SIP ACK to the proxy
server. The ACK confirms that User A
has received the 200 OK response. The
call session is now active.
The proxy server sends the SIP ACK to
User B. The ACK confirms that the proxy
Description
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.