Yealink SIP-T2 SERIES Administrator's Manual page 1031

Step
180 Ringing—Proxy Server to
F12
User A
200OK—User C to Proxy
F13
Server
200OK—Proxy Server to User
F14
A
F15 ACK— User A to Proxy Server
F16 ACK—Proxy Server to User C
The following figure illustrates successful 3-way calling between Yealink IP phones in
which User A mixes two RTP channels and therefore establishes a conference between
User B and User C. 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 places User B on hold.
4. User A calls User C.
5. User C answers the call.
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 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 sends the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response.
Appendix
Description
1009