Yealink T46G Administrator's Manual page 418

Yealink Skype for Business HD IP Phones Administrator Guide
Step
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
404
Action
notifies User A that the connection has
been made.
User A sends a SIP ACK to the proxy server.
The ACK confirms that User A has received
the 200 OK response. The call session is
now active.
The proxy server sends the ACK message to
User C. The ACK confirms that the proxy
server has received the 200 OK response.
Description