Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
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180 Ringing—Proxy Server to
User A
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.
200OK—User C to Proxy Server
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.
200OK—Proxy Server to User A
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.
ACK— User A to Proxy Server
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.
ACK—Proxy Server to User C
The proxy server sends the ACK message
to User C. The ACK confirms that the
proxy server has received the 200 OK
response.
Call Conference
The following figure illustrates successful three-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.
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