Yealink Skype for Business HD IP Phones Administrator Guide392Step Action DescriptionINVITE was successfully processed.F16 200 OK—Proxy Server to User AThe proxy server forwards the 200 OKresponse to User A. The 200 OK responsenotifies User B is successfully placed onhold.F17 ACK—User A to Proxy ServerUser A sends an ACK message to the proxyserver. The ACK confirms that User A hasreceived the 200 OK response. The callsession is now temporarily inactive. No RTPpackets are being sent.F18 ACK—Proxy Server to User BThe proxy server sends the ACK message toUser B. The ACK confirms that the proxyserver has received the 200 OK response.F19 200 OK—User A to Proxy ServerUser A sends a 200 OK response to theproxy server. The 200 OK response notifiesthat the connection has been made.F20 200 OK—Proxy Server User C The proxy server forwards the 200 OKmessage to User C.F21 ACK—User C to Proxy ServerUser C sends a SIP ACK to the proxy server.The ACK confirms that User C has receivedthe 200 OK response. The call session isnow active.F22 ACK—Proxy Server to User AThe proxy server forwards the SIP ACK toUser A to confirm that User C has receivedthe 200 OK response.Call Transfer without 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 a third partywithout consultation. This is called a blind 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 B transfers the call to User C.4. User C answers the call.
