Appendix831Step Action DescriptionServer indicates that the user is being alerted.F10 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 alerted.F11 200OK—User C to Proxy ServerUser C sends a SIP 200 OK response to theproxy server. The 200 OK response notifiesUser A that the connection has been made.F12 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.F13 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.F14 ACK—Proxy Server to User CThe proxy server forwards the ACK messageto User C. The ACK confirms that the proxyserver has received the 200 OK response.The call session is now active.Busy Call ForwardThe following figure illustrates successful call forwarding between Yealink SIP IP phones in whichUser B has enabled busy call forward. The incoming call is forwarded to User C when User B isbusy. 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 B enables busy call forward, and the destination number is User C.2. User A calls User B.3. User B is busy.4. User B forwards the incoming call to User C.5. User C answers the call.
