Yealink Skype for Business HD IP Phones Administrator Guide388Step Action Descriptionnow active.F8 ACK—Proxy Server to User BThe proxy server sends the SIP ACK to UserB. The ACK confirms that the proxy serverhas received the 200 OK response. The callsession is now active.F9 INVITE—User A to Proxy ServerUser A sends a mid-call INVITE request tothe proxy server with new SDP sessionparameters, which are used to place the callon hold.F10 INVITE—Proxy Server to User B The proxy server forwards the mid-callINVITE message to User B.F11 200 OK—User B to Proxy ServerUser B sends a SIP 200 OK response to theproxy server. The 200 OK response notifiesUser A that the INVITE is successfullyprocessed.F12 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.F13 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.F14 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.Successful Call Setup and Call WaitingThe following figure illustrates a successful call between Yealink Skype for Business phones inwhich two parties are in a call, one of the participants receives and answers an incoming callfrom a third party. In this call flow scenario, the end users are User A, User B, and User C. Theyare all using Yealink Skype for Business phones, which are connected 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 C calls User B.
