Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones1092Step Action Descriptionalerted.F12 180 Ringing—Proxy Server toUser AThe proxy server forwards the 180 Ringingresponse to User A. User A hears the ring-back tone indicating that User C is beingalerted.F13 200OK—User C to Proxy ServerUser C sends a SIP 200 OK response tothe proxy server. The 200 OK responsenotifies User A that the connection hasbeen made.F14 200OK—Proxy Server to User AThe proxy server forwards the SIP 200 OKresponse to User A. The 200 OK responsenotifies User A that the connection hasbeen made.F15 ACK— User A to Proxy ServerUser A sends a SIP ACK to the proxyserver. The ACK confirms that User A hasreceived the 200 OK response. The callsession is now active.F16 ACK—Proxy Server to User CThe proxy server sends the ACK messageto User C. The ACK confirms that theproxy server has received the 200 OKresponse.Call ConferenceThe following figure illustrates successful three-way calling between Yealink IP phones in whichUser A mixes two RTP channels and therefore establishes a conference between User B andUser C. In this call flow scenario, the end users are User A, User B, and User C. They are all usingYealink 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.
