Appendix395Step Action DescriptionServer indicates that the user is being alerted.F20 180 Ringing—Proxy Server toUser AThe proxy server forwards the 180 Ringingresponse to User A. User A hears thering-back tone indicating that User C isbeing alertedF21 200OK—User C to Proxy ServerUser C sends a SIP 200 OK response to theproxy server. The 200 OK response notifiesthe proxy server that the connection hasbeen made.F22 200OK—Proxy Server to User A The proxy server forwards the SIP 200 OKresponse to User A.F23 ACK— User A to Proxy ServerUser A sends a SIP ACK to the proxy server.The ACK confirms that User A has receivedthe 200 OK response. The call session isnow active.F24 ACK—Proxy Server to User CThe proxy server forwards the ACK messageto User C. The ACK confirms that User A hasreceived the 200 OK response. The callsession is now active.Call Transfer with ConsultationThe following figure illustrates a successful call between Yealink Skype for Business phones inwhich two parties are in a call and then one of the parties transfers the call to the third partywith consultation. This is called consultative transfer. In this call flow scenario, the end users areUser A, User B, and User C. They are all using Yealink Skype for Business phones, which areconnected 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.
