Yealink SIP-T54S Administrator's Manual page 1109

Step Action
F19 200OK—User C to Proxy Server
F20 200OK—Proxy Server to User A
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
F23 REFER—User A to Proxy Server
202 Accepted—Proxy Server to
F24
User A
F25 REFER—Proxy Server to User B
202 Accepted—User B to Proxy
F26
Server
F27 BYE—User A to Proxy Server
F28 BYE—Proxy Server to User B
F29 200OK—User B to Proxy Server
Description
alerted.
User C 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 SIP 200 OK
response 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 forwards the ACK
message to User C. The ACK confirms that
the proxy server has received the 200 OK
response. The call session is now active.
User A sends a REFER message to the
proxy server. User A performs a transfer of
User B to User C.
The proxy server sends a SIP 202
Accepted response to User A. The 202
Accepted response notifies User A that
the proxy server has received the REFER
message.
The proxy server forwards the REFER
message to User B.
User B sends a SIP 202 Accept response to
the proxy server. The 202 Accepted
response indicates that User B accepts the
transfer.
User A terminates the call session by
sending a SIP BYE request to the proxy
server. The BYE request indicates that User
A wants to release the call.
The proxy server forwards the BYE request
to User B.
User B sends a SIP 200 OK response to the
Appendix
1083