Appendix421Step Action DescriptionF18 INVITE—Proxy Server to User C The proxy server maps the SIP URI in theTo field to User C.F19 180 Ringing—User C to ProxyServerUser C sends a SIP 180 Ringing responseto the proxy server. The 180 Ringingresponse indicates that the user is beingalerted.F20 180 Ringing—Proxy Server toUser AThe proxy server forwards the 180Ringing response to User A. User A hearsthe ring-back tone indicating that UserC is being alertedF21 200OK—User C to Proxy ServerUser C sends a SIP 200 OK response tothe proxy server. The 200 OK responsenotifies the proxy server that theconnection has been 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 proxyserver. The ACK confirms that User A hasreceived the 200 OK response. The callsession is now active.F24 ACK—Proxy Server to User CThe proxy server forwards the ACKmessage to User C. The ACK confirmsthat User A has received the 200 OKresponse. The call session is now active.Call Transfer with ConsultationThe following figure illustrates a successful call between Yealink SIP IP phones in whichtwo parties are in a call and then one of the parties transfers the call to the third partywith consultation. This is called attended transfer. In this call flow scenario, the end usersare User A, User B, and User C. They are all using Yealink SIP IP 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.
