Yealink SIP-T2XP Administrator's Manual page 441

Step Action
180 Ringing—User C to Proxy
F17
Server
180 Ringing—Proxy Server to
F18
User A
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
F24
to User A
F25 REFER—Proxy Server to User B
202 Accepted—User B to
F26
Proxy Server
Description
sends the INVITE request to User C.
User C 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
C is being 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
Appendix
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