Yealink SIP-T48G Administrator's Manual page 292

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
Step
User C
INVITE—User A to Proxy
F13
Server
INVITE—Proxy Server to User
F14
B
200 OK—User B to Proxy
F15
Server
200 OK—Proxy Server to User
F16
A
F17 ACK—User A to Proxy Server
F18 ACK—Proxy Server to User B
200 OK—User A to Proxy
F19
Server
F20 200 OK—Proxy Server User C
F21 ACK—User C to Proxy Server
F22 ACK—Proxy Server to User A
280
Action
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 INVITE was successfully
processed.
The proxy server forwards the 200 OK
response to User A. The 200 OK
response notifies User B is successfully
placed on hold.
User A sends an ACK message to the
proxy server. The ACK confirms that
User A has received the 200 OK
response. The call session is now
temporarily inactive. No RTP packets
are being sent.
The proxy server sends the ACK
message to User B. The ACK confirms
that the proxy server has received the
200 OK response.
User A sends a 200 OK response to the
proxy server. The 200 OK response
notifies that the connection has been
made.
The proxy server forwards the 200 OK
message to User C.
User C sends a SIP ACK to the proxy
server. The ACK confirms that User C
has received the 200 OK response. The
call session is now active.
The proxy server forwards the SIP ACK
Description