Appendix811Step Action Descriptionindicates that the INVITE request has beenreceived by User B.F4 100 Trying—Proxy Server to UserAThe proxy server forwards the SIP 100Trying to User A to indicate that the INVITErequest has already been received.F5 486 Busy Here—User B to ProxyServerUser B sends a SIP 486 Busy Here responseto the proxy server. The 486 Busy Hereresponse is a client error responseindicating that User B is successfullyconnected but User B is busy on the IPphone and unable or unwilling to take thecall.F6 486 Busy Here—Proxy Server toUser AThe proxy server forwards the 486 BusyHere response to notify User A that User Bis busy.F7 ACK—User A to Proxy ServerUser A sends a SIP ACK to the proxy server.The SIP ACK message indicates that User Ahas received the 486 Busy Here message.F8 ACK—Proxy Server to User BThe proxy server forwards the SIP ACK toUser B to indicate that the 486 Busy Heremessage has already been received.Unsuccessful Call Setup—Called User Does Not AnswerThe following figure illustrates the scenario of an unsuccessful call caused by the called user’s noanswering. In this scenario, the two end users are User A and User B. User A and User B arelocated at Yealink SIP IP phones.The call flow scenario is as follows:1. User A calls User B.2. User B does not answer the call.3. User A hangs up.
