Yealink SIP-T2 SERIES Administrator's Manual page 1017

Step
180 Ringing—Proxy Server to
F20
User A
200OK—User C to Proxy
F21
Server
200OK—Proxy Server to User
F22
A
F23 ACK— User A to Proxy Server
F24 ACK—Proxy Server to User C
The following figure illustrates a successful call between Yealink SIP IP phones in which
two parties are in a call and then one of the parties transfers the call to the third party
with consultation. This is called attended transfer. In this call flow scenario, the end users
are User A, User B, and User C. They are all using Yealink SIP IP phones, which are
connected via an IP network.
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User A calls User C.
4. User C answers the call.
5. User A transfers the call to User C.
Action
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 the proxy server that the
connection has been made.
The proxy server forwards the SIP 200
OK response to User A.
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 User A has received the 200 OK
response. The call session is now active.
Appendix
Description
995